Autofocus system for scanning laser inspector or writer

ABSTRACT

A laser pattern inspection and/or writing system which writes or inspects a pattern on a target on a stage, by raster scanning the target pixels. An autofocus keeps the scanning laser beam in focus on the target. The autofocus system includes an objective lens assembly through which a first laser beam is directed onto a writing surface, an autofocus laser which is detected by a photodetector after it is reflected from the surface of a target, and a linear magnetic motor for moving a lens in the objective lens assembly to keep the first laser beam focused on the writing surface.

This application is related to the following applications assigned toTexas Instruments Incorporated: TI-13385, TI-13386, TI-13387, TI-12390,TI-12961, TI-12960, TI-12958, TI-12957, TI-12386, TI-12387, TI-12389,TI-13403, TI-12391, TI-12392, TI-12393, TI-12394, TI-12326, TI-12388,TI-12959, TI-1338, TI-13402, Tl-12385; which by reference areincorporated herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains to laser writers for exposing patterns onlight-sensitive photoresist on semiconductor wafers, printed circuitboards, photomasks, hybrid circuit boards, ceramic boards and the like,and for exposing patterns on light-sensitive film or glass and the like.

2. Description of the Related Art

Many areas of technology are today heavily dependent on the availabilityof semicustom and custom integrated circuits, such as gate arrays andASICs (Application Specific Integrated Circuits). Long cycle time in thedevelopment of these key components frequently results in componentsthat have not been optimized adequately and that have been veryexpensive to develop. Extensive computer simulation is required toreduce the number of redesigns. Even with this simulation, systemvalidation requires actual devices for in-circuit testing. Better systemperformance often can be obtained with less simulation by the earlyavailability of low cost prototype integrated circuit devices.

Effective prototyping of integrated circuit devices requires several keyfeatures. Initially, effective design capture is required. Thiscapability has been broadly implemented in the industry with tools suchas CAD/CAM. The initial data checking and validation tools are in placeand broadly used. Standard design flows have been established from thisdesign capture area to the patterning data bases. These flows canusually be executed quickly, often resulting in overnight delivery ofthe plot data base.

Frequently, long delays occur at this point. The conventional approachis to fabricate and qualify a standard set of production, high volumephotomasks at a high cost, and days to weeks of processing, queing, andqualification time. When the actual semiconductor wafers are to havetheir photoresist exposed, processing capacity is significantly reducedby the downtime required for loading, aligning, and calibrating each newreticle, for example. This can be as quick as one minute or, morecommonly, as long as 15 minutes. Thus, the capacity of a critical pieceof equipment such as a wafer stepper can drop below four wafers perhour. From the standpoint of assured integrated circuit devicecompletion, it is desirable to write more than one wafer. If only oneintegrated circuit device design is used per wafer, the result can bebuilding more than 1200 devices for a prototype test run of threewafers. If more than one integrated circuit design is used per wafer,then the reticle loading problems become even more severe. The endresult is very expensive, long turnaround prototype integrated circuitdevice fabrication. The severity of the problem increases as the numberof custom features in an integrated circuit device increases. Thus,semi-custom devices such as gate arrays commonly are fabricatednotwithstanding these difficulties, but prototyping of fully customdevices is usually restricted to long production runs or to very highvalue products.

An alternative to photolithographic steppers is direct e-beampatterning, which has been applied in a limited number of cases. E-beampatterning has proven effective where small quantities of very highvalue devices are being built. An advantage to e-beam machines is thatit can be practical to fabricate multiple integrated circuit designs ona single wafer since no physical mask or reticle is required. E-beammachines are, however, very costly and slow, resulting in patterningcosts that are not acceptable for production runs, once the prototypedevices have been produced. Furthermore, e-beam machines have a poorrecord of uptime, which further restricts their use forphotolithographic writing on wafers. Another disadvantage to e-beamwafer writers is that they require the use of special electron beamsensitive resists. These resists have not shown the chemical durabilityor freedom from defects known as pinhole defects, obtainable with modernoptically sensitive photoresists. Thus, e-beam masks require inspectionand frequently require repair to achieve acceptable defect levels. Withresist coated wafers, however, this type of repair is not physicallypractical.

Printed wiring board fabrication has the same types of patterningproblems as those of integrated circuit patterns on wafers, but on adifferent scale. Printed wiring boards may be exposed by writingdirectly on photoresist covering the surface of a board, using a beam oflight or electrons. Alternatively, exposure can take place by projectingpatterns of light and dark on the surface of a board using a glass plateor film artwork.

In addition to the same problems already mentioned above concerninge-beam use for direct patterning on wafers, e-beams could not be used toproduce patterns on printed wiring boards much above six inches by sixinches. For printed wiring boards greater than this size, glass plate orfilm artwork or masks would have to be used. It should be noted,however, that e-beam machines normally are not used to produce printedwiring boards and such use is only hypothetical. The standard printedwiring board mask manufacturing machine has been Gerber, which usesphotographic exposure through various apertures to describe thegeometries.

Current systems for creating printed wiring board masks suffer fromproblems of poor fabrication cycle time, storage and maintenance ofmaster artwork over a period of years. Additionally, the artwork must becarefully inspected for errors, adding to the cycle time.

Attempts have been made to produce a laser writer capable of writingdirectly on photoresist or creating printed wiring board artwork, usingrotating mirror technology. Rotating mirror technology has been used toproduce laser character printers, however, in the field of artworkgeneration and direct writing on printed wiring boards, the technologyhas some serious drawbacks.

Prior art rotating mirror scanner laser writers for printed wiring boardapplications produce what is basically a circular scan. For thesesystems to write at uniform velocity, an F-theta lens is required. Thefield correction provided by this lens is incomplete. If reasonablyaccurate laser beam positioning on a target is to be achieved,additional levels of position correction are required. Some of thesecorrection techniques include the use of a separate pilot laser positionsensing beam monitored by a CCD line scanner, or the equivalent. Othercorrection methods include the use of galvanometer mirror systems toapply corrections. All of these methods of correction result inincomplete correction that limits the accuracy and resolution of thesesystems.

Another source of errors inherent in the use of rotating mirror systemsis the multifaceted mirror assembly itself. Small mirror assemblies arelimited in the number of reflecting sides that can be ground on theassembly, and still maintain the necessary accuracy.

Larger mirror assemblies may be used to correct the problems inherent insmall mirror assemblies, however, larger rotating mirror assembliessuffer from distortion caused by forces generated by the necessary largerotational velocities.

Mirror assemblies also suffer from problems of balance and the supportbearing performance.

As a consequence of the problems inherent in rotating mirror writersystems, the systems are limited in writing speed, accuracy andstability by the mirror assembly problems mentioned above.

Another attempt to produce a laser writing system uses anacousto-optical Bragg cell as a laser beam deflector. This system isdisclosed in U.S. Pat. Nos. 4,541,712 and 4,620,288. What is disclosedis a laser-based pattern generating system for writing patterns onlight-sensitive material to produce reticles and for direct on-waferpattern generation. This system uses a beam splitter to split a laserbeam into an array of sixteen parallel beams, which is swept by theBragg cell from side-to-side in raster-scan fashion to form swaths ofsixteen parallel rows of modulated-light-exposed strips on the surfaceof a target such as a reticle or wafer. The swaths are laid down inparallel rows contiguous to one another to build up, swath-at-a-time, adesired image on the target surface.

After the 16 laser beams pass through the beam splitter they areindividually modulated using acousto-optical modulators. The beams arethen recombined in an attempt to form an array of precisely spaced,parallel beams which must strike the surface of the target in aprecisely spaced pattern. This is required to achieve accuratelyparallel lines and to prevent wave interference effects between thebeams.

After the 16 beams are recombined they are deflected by the Bragg celldeflector in raster scan fashion to sweep across the surface of thetarget. The acousto-optical bragg cell for deflecting the 16 recombinedbeams has a sweep rate dependent upon the frequency slew rate of theoscillator producing acoustic waves travelling transversely to theoptical axis in the deflector. Linearity of slew rate is very difficultto achieve, which means that linearity in the sweep of the laser arrayis impossible to achieve for all practical purposes.

SUMMARY OF THE INVENTION

An autofocus system for a pattern inspection or writing system,comprising: an objective lens assembly for focusing a first laser beamon the surface of a target for pattern inspection or writing; anautofocus laser fixed to the objective lens assembly, for directing asecond laser beam to strike the surface of the target at an obliqueangle; a photodetector for detecting the relative movement of thereflected second laser beam, as the distance of the objective lensassembly to the surface of the target varies, and for generating anelectrical signal in response to the relative movement; and aservo-system responsive to the electrical signal, for moving a firstobjective lens axially to keep the first laser beam focused on thesurface of the target as the distance of the objective lens assembly tothe surface of the target varies.

The autofocus system, wherein the first objective lens is a constantmagnification, variable focus objective lens.

The autofocus system, wherein the first objective lens is included inthe objective lens assembly.

The autofocus system, wherein the photodetector is fixed to theobjective lens assembly.

The autofocus system, wherein the photodetector is a split, Double-D,silicon photodetector.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram, isometric view of an embodiment of the laserpattern writer or inspector writing or inspecting a pattern on a target.

FIG. 2 is a plan view of a target.

FIG. 3 is a plan view of an 18×24 inch printed wiring board targethaving photoresist on its upper face, with an expanded view of a frame.

FIG. 4 is a plan view of a target with illuminated pixels shown forproducing a triangle, also shown.

FIG. 5 is a block diagram showing the laser and optical system of thelaser pattern writer or inspector.

FIG. 6 is a block diagram of a laser pattern writer.

FIG. 7 is a diagram showing ideal and actual pulse trains from theautocompensator.

FIG. 8 is a flow chart illustrating a smoothing filter algorithm.

FIG. 9 is a data flow diagram for the pattern writer system.

FIG. 10 is a table illustrating the contents of Main Directory Record 1of the tape resident database.

FIG. 11 is a table illustrating the contents of Main Directory Record 2of the tape resident database.

FIG. 12 is a table illustrating the content of Target Header Record 1 ofthe tape resident database.

FIG. 13 is a plan view of a target showing a pattern and its constituentpolygons.

FIG. 14 is a plan view of an allowed rectangular polygon.

FIG. 15 is a plan view of an allowed trapezoidal polygon with parallelupper and lower sides and a vertical left side.

FIG. 16 is a plan view of an allowed trapezoidal polygon with parallelupper and lower sides and a vertical right side.

FIG. 17 is a plan view of an allowed parallelogram polygon withhorizontal top and bottom sides.

FIG. 18 is a plan view of an allowed trapezoidal polygon with paralleltop and bottom sides.

FIG. 19 is a plan view of an allowed trapezoidal polygon with parallelleft and right sides and a horizontal top side.

FIG. 20 is a plan view of an allowed trapezoidal polygon with parallelleft and right sides and a horizontal bottom side.

FIG. 21 is a plan view of an allowed parallelogram polygon with verticalleft and right sides.

FIG. 22 is a plan view of an allowed trapezoidal polygon with parallelleft and right sides.

FIG. 23 is a table showing the bit positions for the control words usedin the tape resident database.

FIG. 24 is a table defining the meaning of the control bits of thecontrol word.

FIG. 25 is a table indicating the structure of the Data Records of thetape resident database.

FIG. 26 is a plan view of one strip and two frames of a target, with andexample rectangular pattern to be written or inspected.

FIG. 27 is a listing showing the tape resident database Data Records forthe example pattern of FIG. 26.

FIG. 28 is a table of the meanings of the various control words used inthe disk resident database.

FIGS. 29 and 30 are listing of the data output by DPC2 for the examplepattern of FIG. 26.

FIG. 31 is a listing of the data output by DPC3 for the example patternof FIG. 26.

FIG. 32 is a listing of the data output by the preprocessors for theexample pattern of FIG. 26.

FIG. 33 is a plan view of one strip and two frames showing the vectorform of the example pattern of FIG. 26.

FIG. 34 is a table indicating the meaning of the command and data wordsfor the vector representation of a pattern.

FIGS. 35 and 36 are listings of the data output by the filler modulesfor the example pattern of FIG. 26.

FIG. 37 is a listing of the bit-mapped data output by the pixel memorymodules for the example pattern of FIG. 26.

FIG. 38 is a block diagram of a pattern inspector system which also hasthe capability of operating in a writer mode.

FIG. 39 is a plan view showing an example of an error in an inspectedpattern in a single strip on a target.

FIG. 40 is a plan view showing a second example of an error in aninspected pattern in a single strip on a target.

FIG. 41 is a side view of the light detection system for the patterninspector system.

FIG. 42 is a plan view of the light detection system for the patterninspector system.

FIG. 43 is a side view illustrating specular and diffuse reflection froma target.

FIG. 44 is a side sectional view of the objective lens and autofocusmechanism for the pattern inspector system.

FIG. 45 is a plan view of a reticle target on a stage, showing top andbottom fiducial marks.

FIG. 46 is a plane view of a section of a semiconductor wafer, includingthe individual IC bars.

FIG. 47 is a plan view showing the rectangular pattern of FIG. 26 withguardbands surrounding its sides.

FIGS. 48A and 48B are listing of the output of DPC2 for the first frameof data for the example pattern of FIG. 47.

FIGS. 49A and 49B are listings of the output of DPC3 for the guardbanddata only, for the example pattern of FIG. 47.

FIGS. 50A and 50B are listings of the output of the preprocessor(s) forthe guardband data only, for the example pattern of FIG. 47.

FIGS. 51A through 51C are listings of the output of the filler module(s)for the guardband data only, for the example pattern of FIG. 47.

FIG. 52 is a listing of the bit-mapped output of the pixel memory(s) forthe guardband data, for the example pattern of FIG. 47.

FIGS. 53A and 53B are data flow diagrams for the pattern inspectionsystem.

DESCRIPTION OF THE PREFERRED EMBODIMENTS PATTERN WRITING

FIG. 1 shows the pattern writer system 50 with a target 103 having aphotosensitive surface 121, mounted on granite stage 118. The target maybe a reticle, a semiconductor wafer with photoresist on its face, aglass plate with a photosensitive surface layer, a photographic film ora printed wiring board with photoresist on its face. In addition, theinvention comprehends use with photosensitive surfaces in general.

An argon ion laser 100 emits laser beam 122 which enters electro-opticalmodulator 101 which modulates laser beam 122 by letting the beam passthrough or blocking it in correspondence to the serial bit patternreceived in serial pixel bit stream 110. Electro-optical modulator 101is purchased from Conoptics Incorporated from Danbury, Conn. A ConopticsModel 100 is used when a modulation rate between 50 MHZ and 100 MHZ isdesired, and a Model 50 is used when 50 Mhz or less is satisfactory. Themodulator 101 is used in the binary output mode of either letting 011incident light pass through or else blocking the light entirely.

After passing through a Bragg cell tracker 113 and beam shaping optics114, 115, shown in FIG. 5, modulated laser beam 123 enters chirpdeflector 102. Chirp deflector 102 is an acousto-optical deflector whichcauses the laser beam 104 to sweep out raster scan lines on thephotosensitive surface 121 of target 103. The surface 103 will have thatportion of its surface raster scanned as requires exposure to light inthe desired geometrical patterns. The surface 121 will be developedafter the desired geometries have been written by the raster scannedlaser beam 104, and the photoresist which was not exposed to the laserbeam will thus be removed to expose the surface beneath the photoresist.Typically, in integrated circuit manufacture, this exposed surface willbe a layer of metal which will be etched away, leaving a metalliccircuit pattern under and defined by the exposed photoresist.

The stage 118 is capable of bi-directional movement along both the Xaxis 125 and the Y axis 124. The stage also is capable of rotationalmovement in the X-Y plane, about an axis perpendicular to the X axis 125and the Y axis 124, to align the target 103 with respect to the X and Yaxes. The chirp deflector 102 produces scanning or deflection in the Xdirection only. Relative Y direction motion between the target 103 andthe scanning laser beam 104 is produced by moving the stage 118 in theplus or minus Y direction. The coordinate system for the target 103 ischosen so that the positive Y direction is indicated by the direction ofY axis arrow 124. This is opposite the conventional way of assigning asign to a Y coordinate. The positive X direction is conventional and isin the direction of X axis arrow 125.

As seen in FIGS. 1, 2 and 3, the target surface 121 is treated bypattern writer system 50 as being partitioned into a plurality ofcontiguous strips 105 of equal width W in the X direction. The sum ofthe individual widths W equals the width of the target 103 in the Xdirection. The height H of each strip 105 is the height of the target103 in the Y direction. Each strip 105 is treated by pattern writersystem 50 as being partitioned vertically into a plurality of contiguousframes 106. Each frame is of height FH in the Y direction, and the sumof the individual frame heights in a strip equals the height H of thetarget 103.

Each frame 106 is treated by pattern writer system 50 as having its areapartitioned into a matrix of contiguous, equal diameter, circular pixels119, as shown in FIG. 3. Preferably, the pixels 119 intersect withadjacent pixels at least one point. The pixels 119 illustrated in FIG. 3are not shown as touching one another for clarity of illustration,although, preferably adjacent pixels would touch each other. Ageometrical shape which is to be written in a frame 106 on surface 121will be written by exposing with laser beam 104 each pixel enclosedwithin the boundary of the geometrical shape. Pixels not containedwithin the boundaries of the geometrical shape will not be exposed tothe laser beam 104. That is, laser beam 104 will be turned on byelectro-optical modulator 101 only at those pixels contained within theboundary of the geometrical shape.

The power distribution of a laser beam striking surface 121 isapproximately Gaussian, with the peak at the center of the area on 121illuminated by the laser beam 104. The diameter of a pixel isapproximately the diameter of the half power circle centered on thepower peak. That is, approximately one half the power delivered by thelaser beam to the surface 121, falls within the pixel. The photoresistsused with this invention become exposed by those portions of theincident laser beam 104 which have sufficient energy to fall within thehalf power circle. It is evident that where adjacent pixels touch onlyat one or a few points, there will be portions of the surface 121 whichdo not lie within any pixels. However, these areas can still be made tohave sufficient laser energy fall upon them to expose the photoresistthere.

This can be understood by considering two adjacent pixels which touchonly at one point. If each pixel is in turn illuminated by laser beam104 by turning on the beam only when it will strike one of the pixels,it is evident that half the power being delivered to a pixel will falloutside the borders of the illuminated pixel. The areas between the twopixels but within neither pixel will receive some laser light when thefirst pixel is illuminated and then again when the second pixel isilluminated. Thus, the energy delivered from the sum of the twoexposures will be sufficient to expose most of the areas lying betweenbut not in adjacent pixels. This feature means that geometrical shapeson surface 121 can be made to contain negligibly few or nogranularities. That is, given a geometrical shape within whoseboundaries all pixels have been illuminated, all of the areas lyingwithin the boundaries will receive enough incident laser light to exposesubstantially all of the photoresist within those boundaries. This istrue even for geometrical shapes which lie in more than one adjacentframe and/or in more than one adjacent strip. There is no more distancebetween adjacent pixels when those pixels lie in different frames orstrips than is the case with adjacent pixels lying in the same frame.

FIG. 4 shows which pixels 119 are illuminated in order to write atriangular shape 129 on photosensitive surface 121. The illuminatedpixels are shown as half-power circles 130 which touch adjacent pixelsat a single point on their half-power circles 130. Adjacent pixels aredefined as being pixels which are next to each other either in the samerow or same column of the matrix of pixels. A pixel is illuminated bylaser beam 104 if the pixel center 128 falls within the boundary of thedesired geometrical shape, in this case, triangle 129. It can be seenthat the aggregate of the illuminated pixels does not form a perfectlyshaped triangle when viewed from a perspective which is magnified to theextent of FIG. 4. However, geometries are rarely as small with respectto the size of an individual pixel as is shown in FIG. 4, thus, theirregularities inherent in forming a shape from constituent pixels arevery small in size with respect to the size of the desired geometricalshape. In the example of FIG. 4, the pixel diameters could be reduced insize and the pixel density thereby increased so that the pattern ofilluminated pixels more closely matches the desired triangle shape 129.In fact, for writing integrated circuit patterns directly onsemiconductor wafers, the diameter of the half-power circle is reducedin size to become 0.5 micron. This is an order of magnitude smaller thanthe half-power circle size of 0.2 Mil (2 Mil=5.08 micron).

The target illustrated in FIG. 3 is a printed wiring board target 103with a photoresist surface 121. The board has a height of eighteeninches and a width of 24 inches, thereby illustrating the large sizeprinted wiring board on whose surface 121 the invention is capable ofwriting circuit geometries. In this example, the pixel 119 size is 0.2Mil in diameter (0.0002 inches). Each strip 105 is 1024 pixels wide,which is also the width of each frame 106, and each frame is 512 pixelshigh. The pixels 119 are arranged in rows and columns. Preferably,adjacent rows and adjacent columns have a single common spacing distanceso that the distance between two adjacent pixels 119 in a row is thesame as the distance between two adjacent pixels 119 in a column. Theillustrated frame has a height of approximately 1 inches and a width ofapproximately 0.2 inches. There are 120 strips (not all shown) acrossthe width of the target and 180 frames (not all shown) along the heightof the target 103. Each frame has 524,288 pixels, and there are on theorder of ten to the ninth pixels for the entire target 103.

As illustrated in FIG. 1 and FIG. 2, laser beam 104 sweeps out lines 120or rows of pixels from left to right across the width of a strip 105.The chirp deflector 102 deflects the laser beam 123 incident upon it andcauses the emerging laser beam 104 to sweep through an angle subtendedby the width W of the strip being written to. As discussed more fully inconjunction with FIG. 5, the angular sweep speed is a function of thevelocity of a series of acoustic waves propagating transversely throughthe chirp deflector 102. The sweep speed is highly linear due to therelative constancy of the velocity of the acoustic waves as they travelthe length of the chirp deflector 102. As the acoustic waves travelthrough the chirp deflector 102, the electro-optical modulator 101allows the laser beam 123 to emerge and enter the chirp deflector whenthe angular deflection produced by the chirp deflector is such that thedeflected laser beam 104 will strike a pixel whose center lies withinthe boundaries of the desired geometrical figure. The electro-opticalmodulator blocks laser beam 123 so that no laser beam 104 is emittedwhen the angle of deflection would otherwise cause laser beam 104 tostrike a pixel which should not be illuminated. A pixel will not beilluminated when its pixel center 128 lies outside of the boundaries ofthe desired geometrical shape, such as the triangle 129 of FIG. 4.

The geometrical patterns which are to be produced on the photosensitivesurface 121 of target 103 are produced by illuminating each pixelcontained within the boundaries of the desired geometrical patterns.Such structures as circuit connections on the face of a printed wiringboard can be constructed geometrically by partitioning the overallgeometrical shape of the circuit structures into a plurality ofconstituent, contiguous polygons. Each such constituent polygon iscontained within a single frame 106. Thus, a geometrical feature whichis large enough, as most practical features will be, to extend over aplurality of frames, will consist of polygons from all the frames 106into which the feature extends.

Each constituent polygon in a frame 106 will have the pixels containedwithin the boundary of that polygon, illuminated by the laser beam 104by raster scanning the area in the frame with modulated laser beam 104.In the embodiments of the invention shown, the pixels in a frame arearranged into 512 rows of 1024 pixels in each row. A frame is rasterscanned by beginning at the left end of the uppermost row in the frameand sweeping modulated laser beam 104 across the full width of the frame106. Thus, each of the 1024 pixels in the top row will be passed over bythe modulated laser beam. The modulated laser beam 104 is turned on whenthe beam is over a pixel to be illuminated, is turned off betweenpixels, and is off when over pixels which are to receive no illuminationbecause their centers 128 lie outside the 104 is turned on only when theraster scan position is such that the laser beam will strike a pixelthat lies within one of the polygon(s) within the frame. While the toprow is being scanned, the stage 118 is continuously moving in thenegative Y direction, carrying target 103 with it. After the top row hasbeen scanned, the stage 118 is allowed to move in the negative Ydirection the distance of one pixel position, and the second row of 1024pixels is scanned by modulated laser beam 104. Again, the stagecontinues to move in the negative Y direction while the second row isbeing scanned. This process continues until all 512 rows of pixels havebeen scanned by the modulated laser beam 104. The stage 118 continuouslymoves during the entire 512 raster scans of the frame, so that there isa slight tilt downward to the right of each of the 512 rows. The pixelscan rate is sufficiently fast (up to 100 MHZ) that for all practicalpurposes a scan of a line happens instantaneously. The speed of thestage in the negative Y direction is slow relative to the scan speed,thus, the slope of the rows is almost imperceptible and causes nosignificant distortion of the patterns produced on surface 121.

A target 103 is scanned, strip 105 at a time, from left to right. Astrip is scanned frame 106 at a time from top to bottom, although,strips can be scanned in serpentine fashion. That is, after a strip 105has been scanned from top to bottom, the next strip 105 to the right canbe scanned from bottom to top. This saves some run time since the stage118 does not have to move in the positive Y direction back to thetopmost frame 106 in the next strip to the right before beginning toscan that strip. As discussed above, frames 106 are scanned row at atime from top to bottom, except when operating in the serpentine mode,in which case frames are scanned row at a time from bottom to top. Rowsare scanned one pixel at a time from left to right. As seen in FIG. 2,raster scanning of target 103 begins in the leftmost strip 126 in theuppermost or top frame. The uppermost frame is scanned one row at timebeginning with the topmost row. The illustrated topmost frame has hadfour rows raster scanned and the fifth row is partially completed.

Raster scanning of a row 120 is accomplished by transmitting acousticpulses into chirp deflector 102 which causes laser beam 104 to scan therow from left to right for the width of the frame 106. The chirpdeflector 102 produces no deflection of the laser beam 104 in the Ydirection. This is produced by the motion of the stage 118 in thenegative Y direction, which shifts the target 103 to the Y position ofthe next row 120. After a strip 105 has been raster scanned, the nextstrip to the right is raster scanned after first moving the stage 118 inthe negative X direction for a distance equal to the width of strip 105.The target 103 will have been completely raster scanned after strip 127has been raster scanned.

X and Y Scan Compensation

The stage 118 and stage controller 111, shown in FIG. 1, are availablefrom the Anorad Corporation of Hauppauge, N.Y. The stage 118 preferablyis solid granite four inches thick and more than eighteen inches highand twenty-four inches wide. For writing patterns on semiconductorwafers or smaller printed wiring boards, the surface dimensions of thestage 118 can be less than eighteen inches by twenty-four inches. Thestage moves on air bearings on a granite base of large dimensions.Granite is used for the stage and base because it can be machined tohave very precise flat surfaces and it is a very stable material withrespect to its thermal expansion characteristics. In addition, the largemass of the granite base dampens vibrations transmitted from thesurrounding environment. The Stage controller includes X stage drive 107and Y stage drive 108. Not indicated in FIG. 1 are the drives for thetaand Z axis movement of the stage. X, Y and Z axis glass scale encodersare included for providing stage position information having aresolution of less than one micron. For more accuracy, plane mirrorlaser interferometers can be used rather than glass scale encoders toprovide stage position information. In situations where extreme accuracyis unnecessary, such as creating artwork or exposing photoresistdirectly on large size printed wiring boards, glass scale encoders arepreferable due to faster speed of operation and lower cost. Depending onthe speed, cost and accuracy requirements, other ways of obtaininginstantaneous stage 118 position information can be used as well. Forexample, resolvers, magnetosyns, rotary encoders or grating projectiontechniques are possibilities.

Interferometer position information can be dynamically corrected fortemperature, humidity and barometric pressure. Corrected interferometeror glass scale encoder Y position data may be used to trigger each row120 scan, thus reducing dependency on constant velocity servoperformance. This ensures correct and equal spacing between rows 120 onthe surface 121 of target 103. The X axis interferometer or glass scaleencoder position information may be used to correct the individual rowscan start time delay. This corrects for X direction positioning error,and ensures that the pixels which should have the same X address, formstraight columns. Since the row 120 laser scan is very fast with respectto the stage 118 motion in the Y direction, the X direction positioningcorrection has no significant effect on the Y direction positioning ofthe row. This can be understood by considering that for a 1024 writtenpixel per row scan system the row scan in the X direction is more thanone thousand times faster than the Y direction mechanical stage speed.Stage controller 111 receives stage movement commands from bit-sliceelectronics 109.

FIG. 5 is an expanded block diagram of the laser raster scan opticalsystem, showing more detail than is shown in FIG. 1. Argon ion laser 100emits an unmodulated output beam which is directed into electro-opticalmodulator 101 by mirrors 131 and 132. Laser 100 may be a krypton ion gaslaser. Selection is based on the required writing wavelength. For mostliquid integrated circuit photoresists, an argon ion laser 100 operatedat 457.9 nanometers wavelength is the preferred choice. Because of itsoperation at lower plasma current than a krypton ion laser at 413.1nanometers, less ultraviolet is generated within the laser plasma tube.Since high intensity ultraviolet output within the plasma tube degradesthe tube windows, longer operating life is expected at the argon 457.9nanometer wavelength of operation. For those applications usingphotoresists that require the shorter wavelength light for properexposure, a krypton laser may be used.

Electro-optical modulator 101 receives a serial pixel bit stream 110from bit-slice electronics 109 and modulates the laser beam, asdiscussed above.

The modulated laser beam is reduced in power by attenuator 112.Attenuator 112 can be a half-silvered mirror which reflects a portion ofthe laser beam, while allowing the remainder of the beam to passthrough. Preferably, however, attenuator 112 is an electricallycontrolled variable attenuator. The attenautor 112 is used to select thepower level required for writing photographic emulsion or differentphotoresists. The power level is usually photoresist takes place withinthe half-power circle 130 of the incident laser beam 104, but does nottake place outside the half-power circle.

The laser beam which emerges from the attenuator is directed to tracker113 by mirror or prism 133. Tracker 113 is used to provide coarse aimingof the laser beam to improve the optical efficiency of the chirpdeflector 102. The laser beam which is input to the tracker 113 isemerges from the tracker as a beam which sweeps in time through an angleon the order of the angle subtended by the aperture of chirp deflector102. Tracker 113 is an acousto-optical Bragg cell which operates in theflooded aperture mode. An RF signal is input to one end of the Braggcell and it travels transversely to the optical axis. The input RF isramped up in fequency so that the angle of deflection of the laser beamemerging from the tracker is dependent upon the frequency of the RF. Thetracking speed of the emergent laser beam is as linear as is the inputRF ramp. On the order of 5% linearity has been found to be sufficient tofunction as a tracker. The input RF is ramped up between two fixedfrequencies. When the high end of the ramp is reached, the input RF isagain lowered to the lower of the two fixed frequencies and again rampedup. The RF input is in fact a saw-tooth wave of a wavelength on theorder of the size of the optical aperture of the Bragg cell. The sweepfrequency of the laser beam output from the tracker is, in fact, thefrequency of the saw-tooth wave. The purpose of the tracker will bediscussed below in conjunction with the chirp deflector 102.

The swept laser beam from the tracker 113 is directed by prism or mirror134 into beam shaping lenses 114 and anamorphic beam expander 115. Thebeam forming lenses and anamorphic beam expander convert the narrowangle scan from the tracker into the wider scan angle required by chirpdeflector 102. Also, the cross-sectional shape of the tracking laserbeam is changed to the elliptical shape which is required for best chirpdeflector 102 efficiency with minimum edge effects. Anamorphic beamexpanders such as the one used in the invention are discussed in thearticle: In-line Anamorphic Beam Expanders, Applied Optics, Vol. 21, No.15, Page 2861, which is hereby incorporated by reference.

The chirp deflector 102 provides the high resolution stable linear laserscan required. The scan is inherently linear depending upon theuniformity of the acoustic pulse in the solid crystal block of highpurity optical material in the deflector. Thus, no scan linearitycorrections are required, resulting in very stable precision geometrypositioning on the surface 121 of target 103. The chirp deflector 102focuses the output scanning laser beam in the X direction, i.e., thedirection of scan. The chirp deflector differs from a Bragg cell in thatthe linearity of the output scan depends only on the constancy of thespeed of acoustical energy in the optical medium of the deflector, whilelinearity of the Bragg cell depends upon the linearity of the ramp ofthe input RF acoustic wave.

Chirp deflector 102 deflects the input laser beam by diffraction. Atrain of acoustic pulses of RF is input to the optical medium of thedeflector and travel transversely to the optical axis. Each acousticpulse or chirp is a short section of an acoustic compression wave of RFand is linearly ramped from a fixed lower frequency to a fixed higherfrequency. The more nearly linear the ramp is, the more nearly focusedthe scanning output laser beam will be. As one acoustic pulse leaves theoptical medium of the chirp deflector, another enters it. Betweenadjacent pulses the amplitude of acoustical energy is zero. The trackinglaser beam which is input to the chirp deflector scans the aperture ofthe deflector at a scan rate matched to the acoustic pulses in the chirpdeflector 102. That is, the tracking laser beam will strike that portionon the optical medium where the acoustic pulse is at the time, and willtrack the pulse across the aperture of the deflector. When a new pulseenters the deflector, the tracking laser beam begins a new sweep or scanand tracks the new pulse. This mode of operating a chirp deflectorcommonly is called the scanning mode, in contradistinction to theflooded mode.

Chirp deflectors and tracking acousto-optical deflectors are discussedin the following articles and one U.S. Patent and are herebyincorporated by reference:

1. U.S. Pat. No. 3,851,951;

2. Acousto-optic Laser Recording, Optical Engineering, January/February1981, Vol. 20, No. 1, Pages 143-149;

3. Acousto-optic Laser Scanning, SPIE, Vol. 169, Laser Printing (1979),Pages 56-59; and

4. Acousto-optic Laser Recording, SPIE, Vol. 175, AirborneReconnaissance IV (1979), Pages 111-123.

The scanning laser beam output from the chirp deflector has been focusedin the X direction by chirp deflector 102, and is then focused in theorthogonal, i.e., Y, direction by cylindrical lens 116.

Mirror 135 directs the focused, scanning laser beam through objectivelens assembly 117 where final focus is achieved prior to striking thephotosensitive surface 121 of target 103.

Bit-slice electronics 109, shown in FIG. 1, includes a plurality ofbit-slice processors for real-time database expansion row 120 by row 120into a bit-map of the geometrical patterns to be written ontophotosensitive surface 121 of target 103. The bit-map is output row byrow 120 as serial pixel bit stream 110 to the electro-optical modulator101. Each bit which is output corresponds to a predetermined pixelposition on surface 121.

The host computer 3 is a Texas Instruments 990/12 system, and is used asan interface between a human operator and the rest of the pattern writersystem 50.

THE ELECTRONICS

FIG. 6 is a block diagram of one of the embodiments of thephotolithographic laser writer. To begin writing on light sensitivematerial, a human operator communicates to the system via a keyboard 1and video display terminal 2, which are connected to the host computer3. The host computer can be selected from many possible computers, butpreferably is a Texas Instruments 990/12 CPU. which executes theoperating system for interfacing the writer to the human operator. Thehost 3 interrogates operator inputs from keyboard 1 and passes controlinformation to the bit-slice processors DPC2 15 and DPC3 14. The host 3also locates the beginning of the database . . .

The host 3 communicates to the host disk controller 7 and host magneticstorage disk 8 via TILINE 36 bit wide chassis bus 46. The disk 8 iswhere the host operating system is stored and offloaded in order for theoperating system to boot up on the host computer 3. Dynamic randomaccess memory is provided for the host 3 to use at host memory 4. Thehost also communicates with host memory 4 over the chassis bus 46.Preferably the host 3, memory 4, host disk controller 7 and chassis bus46 are contained within a single chassis, although this is not necessaryfor the proper functioning of the invention.

The host 3 is in communication with TILINE 36 bit wide bus 47 by way ofbus buffers 5 and 6, 36 bit buses 48 and 49, and bus buffers 10 and 11.Bus buffer 5 is connected to bus buffer 10 through bus 48, and busbuffer 6 is connected to bus buffer 11 through bus 49. Commands and dataare passed between the host 3 and the bit slice processors DPC2 and DPC3through host memory 4. The host 3 writes commands either to DPC2 15 orDPC3 14 by first writing the command to host memory at a determinedaddress. The host then addresses DPC2 or DPC3 directly to pass thecommand's address in host memory 4 and the length of the command addressto the addressed bit-slice processor. The command itself, residing inhost memory 4, is then read by the addressed bit-slice processor overbus 48. Subsequently, the command is acted upon by the addressedbit-slice processor, and any data resulting from the execution of thecommand is then written over bus 48 and stored at a predetermined memorylocation in host memory 4. An interrupt is then generated by theaddressed bit-slice processor, and is written to the host 3 over bus 48or bus 49. Bus 49 is used solely for passing interrupts from the bitslice processors to the host, and from the data disk controller 12 andthe tape controller 16 to the host 3.

The writer system 50 database is stored on tape at tape drive 17. Thedatabase comprises sixteen bit word, coded descriptions of the polygonsnecessary to describe the geometries which are to be written on aphotosensitive surface. The database is organized into a number ofnon-overlapping contiguous strips and each strip strips and frames ofthe database correspond to the physical strips and frames, shown inFIGS. 1-4, ultimately written on a photosensitive surface. Preparatoryto writing on the photosensitive surface, bit slice processor DPC2 15reads the coded descriptions of the polygons from tape drive 17, recodesthe descriptions into a turnpoint polygon representation, and stores therecoded descriptions of the geometries onto data disk 13, as sixteen bitwords. DPC2 writes control commands over bus 47 to tape drive controller16 to effect the reading of the tape 17, and writes control commandsover bus 47 to data disk controller 12 to effect the storage on the datadisk 13 of the recoded polygon descriptions. During recoding, thepolygon description data is transferred via bus 47 from the tape 17 toDPC2 15, where the recoding process takes place. The recodeddescriptions are then transferred over bus 47 to data disk 13. Theentire database on tape 17 is recoded and transferred to data disk 13before any further processing is done on the data now residing on datadisk 13, and may be done at any time prior to beginning the process ofpattern writing. This allows databases for a number of differentcircuits to be recoded and stored on data disk 13. Thus, the runtime ofthe writing process can be shortened and multiple copies of the samecircuit can be written on printed wiring boards, reticles or photomasks,without having to re-do the process of recoding and transferring thedatabase from tape 17 to data disk 13. In the case of integrated circuitpatterns on multiple dies of a single wafer, the database describes thegeometry of the entire wafer, and thus the process of recoding andtransfer of data from tape 17 to data disk 13 need only be done once fora wafer, even where more than one kind of circuit is to be written onthe same wafer.

Preferably, however, a strip buffer may be used which would, on areal-time basis, store the bit-map for an entire strip which has aheight approximately equal to the wafer height. With this embodiment,separate databases would be maintained only for each unique integratedcircuit pattern. Thus, for a wafer in which all the IC bars, i.e., ICchips, are to have the same integrated circuit pattern applied to each,only one integrated circuit pattern database need be stored on tape ordisk, and subsequently be expanded in the pipelines. The bit map for oneIC bar simply is replicated in the strip buffer for as many IC bars asthe wafer is high.

Further processing of the data now residing on the data disk 13 takesplace in realtime during the process of writing patterns on aphotosensitive surface. DPC2 and DPC3 are each implemented with a 16bit, Texas Instruments SN74S481 bit-slice processor for maximum speed,however, a commercially available microprocessor such as a Motorola68000, a Motorola 68020 or an Intel 80286 could be used instead.

DPC memory 9 is dynamic random access memory for use by DPC2 15 and DPC314, and is accessed by DPC2 and DPC3 over bus 47.

The sixteen bit word, turnpoint polygon data stored on data disk 13 isread by DPC3 14 over bus 47, and is recoded into 25 bit word turnpointpolygon data, with the addition of 25 bit command words interspersedthroughout the data. DPC3 has an output out1 at 51 to a first pipeline,and a second output out2 at 52 to a second pipeline. The two pipelinesoperate in parallel downstream from DPC3 14. DPC3 sends the first frameof recoded, 25 bit data down the first pipeline via out1 51, and sendsthe second frame of recoded, 25 bit data down the second pipeline viaout2 52. Subsequent frames continue to be sent down alternate pipelines,so that contiguous frames of data are never sent down the same pipeline.This increases the speed of processing the data downstream of DPC3 sincecontiguous frames of data are processed in parallel downstream, indifferent pipelines. The processing of any single frame of data thattakes place at DPC3 is much faster than the processing of that frametaking place downstream of DPC3, thus, by multiple pipelines, theeffective speed of downstream data processing is increased. FIG. 6illustrates an embodiment of the invention with two pipelines, whileFIG. 38 illustrates an embodiment of the invention having fourpipelines. The invention also comprehends having other numbers ofpipelines not illustrated. The number of pipelines desired is determinedby the speed of operation desired, and by the relative speeds of theprocessing taking place at DPC3 and downstream from DPC3. The relativespeeds are influenced by the speed of the circuit components used toconstruct the writer system 50, and by the algorithms being executed bythe various processors in the system 50.

Pipeline buffer 18 includes two 4K×25 bit FIFOs, i.e.,first-in-first-out memory, which provides memory buffering forinterfacing the faster DPC3 with the slower downstream processing.

Window clipper 19 can be used to view a pictorial representation ofsegments of the database on color display 29. Since the display 29 canshow only half a physical frame 106, the window clipper must transformmultiple frames of data into a single frame of data. This must be donewhere the area which a human operator wishes to view straddles parts ofmore than one frame of data. The window clipper 19 includes a 24bit-slice processor which executes a window clipping algorithm.Normally, however, the display is not used during pattern writing onphotosensitive material and the 25 bit wide data is simply passedthrough the window clipper without alteration.

Preprocessor1 20A and preprocessor2 20B each include a 16 bit bit-slicemicro-coded processor which recodes the 25 bit word, turnpoint polygondata into 22 bit words in which the upper 6 bits is command informationand the lower 16 bits is data. The turnpoint polygons are recoded intoleft and right vectors. The 16 bit data includes the origin of eachvector, the vector length and the direction of the vector. The twopreprocessors are each a part of a different one of the two pipelines.

The 22-bit-word-data output from preprocessor1 20A is written to fillermodule1 23A, and the 22-bit-word-data output from preprocessor 20B iswritten to filler module2 23B. Filler module1 and filler module 2 eachrecode the 22-bit-word-data from preprocessor1 and preprocessor2,respectively, into 25-bit-word-data and output it to pixel memorymodule1 24A and pixel memory module2 24B, respectively. Each 25 bit worduses the upper 5 bits as a command, the next 10 bits refers to theY-address of a line of data, and the least significant 10 bits refers tothe starting X-address of a group of pixels to be turned on. The size ofthe group of pixels to be turned on is specified in the upper 5 bitcommand word. Each pixel which is turned on corresponds to an X-Ylocation on the photosensitive material which is to be exposed to laserlight. A pixel is the smallest unit of area on the photosensitivesurface which can be illuminated by the laser beam. Its diameter is thediameter of the laser beam at the photosensitive surface. The meaning ofthe X and Y addresses and of the pixels is more fully discussed inconjunction with FIGS. 3 & 4. A Texas Instruments SN74S481 bit-sliceprocessor is included as part of each filler and is microcoded to do therecoding.

The 25-bit-word-data output to pixel memory module1 24A and pixel memorymodule2 24B by filler module1 23A and filler module2 23B. respectively,is recoded by each pixel memory module into 64 bit words. Each bitposition of a 64 bit word corresponds to a pixel and to an X-Y positionon a photosensitive surface. If a bit position is a one, the pixel is tobe turned on. That is, the X-Y position corresponding to that bitposition is to have the laser beam shined on it. If the bit position isa zero, the pixel is to be turned off, and no light will be shined onthe corresponding X-Y position. Each pixel memory module includes aSN74LS25117 arithmetic logic unit microcoded to do the recoding of theincoming data int.o the 64-bit-word data, which then is sent to thehigh-speed laser interface module 27.

High-speed laser interface module 27 selects the output from pixelmemory module1 24A for the first frame's worth of 64-bit-word data, andthen selects the output from pixel memory module2 24B for the secondframe's worth of 64-bit-word data. For subsequent frames of data, theoutputs of pixel memory module1 24A and pixel memory module2 24Bcontinue to be alternately selected, frame-at-a-time, for input tohigh-speed laser interface module 27. Thus, the data generated from thedatabase leaves the two separate pipelines and is recombined in thehigh-speed laser interface module 27.

The high-speed laser interface module 27 includes a counter which countsa frame's worth of bits in order to determine when an entire frame ofdata has been read in. This is possible since each bit which is outputfrom the pixel memory modules represents one pixel, and the total numberof pixels in a frame is fixed at 1024*512=524,288 for this embodiment ofthe invention. Therefore, the high-speed laser interface module 27 readsthe first 524,288 bits of data from pixel memory module1 24A, reads thenext 524,288 bits from pixel memory module2 24B, reads the third 524,288bits from pixel memory module1 24A, and so on, continuing to alternatebetween pixel memory modules for each subsequent frame of data. It isunderstood that other frame sizes could be used without departing fromthe invention.

The high-speed laser interface module 27 also includes a memory forreceiving the 64-bit-word data input to the module. Each 64 bit word isdivided into four contiguous groups of 16 bits. The four groups areoutput one group at-a-time to ECL high-speed laser interface module 28.The four groups are output in descending order from most significant toleast significant.

Each group of 16 bits input by the ECL high-speed laser interface module28 is sent to a 16 bit shift register. There, the bits are shifted outone-at-a-time to form a serial stream of bits, output in descendingorder from most significant to least significant. Each bit position inthe serial stream corresponds to the X-Y position of a pixel on thephotosensitive surface upon which geometrical patterns are beingwritten. The serial stream of bits is output to data level shifter 32,where the logic levels are shifted from ECL levels -1.6 volts and -8volts, to +0.5 volts and -5 volts dc, respectively. From data levelshifter 32, the serial pixel bit stream 110 is output to the laserelectronic and optical system 37. The laser electronic and opticalsystem includeselectro-optical modulator 101 which inputs serial pixelbit stream 110 and modulates the laser beam 122 for writing geometricalpatterns on the photosensitive surface 121.

High speed laser interface module 27 sends expanded database part dataon data monitor path 54 to be displayed on color display 29 for reviewby the human operator. The operator can view what the ideal part on thedatabase should look like at specific locations on an ideal target. Asdiscussed previously, window clipper 19 combines data from multipleframes 106 when the area of the part to be viewed straddles more thanone frame 106, and allows the data to be handled as if it came from asingle frame. The data outside the viewing area of the display isclipped off.

High speed laser interface module 27 sends error code information backalong data monitor path 54 and data return path 55 to DPC3 14. The errorcode information is generated by various modules in the pipelines and isused to ensure that the modules are functioning correctly. DPC3 relaysthe error information to the host CPU 3 which can take correctiveactions, such as aborting the pattern writing process.

The X and Y axis blocks indicated at 34 and 33, respectively, includeprocessor or microprocessor controls for the X and Y glass scaleencoders (or laser interferometers), respectively. Also, X and Ydirection stage 118 move commands are received at blocks 34 and 33,respectively, from stage controller 25. At block 36 theta axis tablerotation commands are received from stage controller 25. The movecommands are translated into control signals which are sent from blocks33, 34 and 36 to stage block 35, where the motors responsible for movingthe stage 118 are actuated by the signals.

Stage block 35 also includes the X and Y glass scale encoders or,alternatively, the X and Y laser interferometers. Stage 118 positiondata is sent from the glass scale encoders to the X and Y axis blockswhere it is sent to autocompensator 31. The autocompensator performsscaling operations on X and Y position pulses received from the glassscale encoders or laser interferometers, as the case may be, tocompensate for temperature and pressure environmental variations.Compensated pulses are transmitted to the smoothing filter 30 and thestage controller 25. Compensation is based upon values received from the990/12 host computer 3 via IEEE-488 bus 136. The compensation values aresupplied in an order specified by the autocompensator firmware. The Xand Y position pulses do not give absolute position information but mustbe counted by stage controller 25 to determine the absolute position ofthe stage 118. Pulse separation corresponds to a predetermined physicaltravel distance for the stage 118, so the sum of all the pulses receivedsince the stage was at rest, is the total distance travelled by thestage. The autocompensator includes 4 bit-slice arithmetic logic units,but could be implemented with a microprocessor instead.

The autocompensator 31 converts the uncorrected position pulse inputfrom either a glass scale stage 35 position encoder or a fast pulseencoder in a laser interferometer system into pulses whose spacecorresponds to the motion of the stage 35 in the mechanical units ofchoice. Specifically, laser interferometers are used to measure stagetravel in units of wavelength of the measuring laser light.

When the measuring path is not in a vacuum, the wavelength of themeasuring light is dependent upon the refractive index of the medium,which in turn is dependent upon the temperature, humidity and barometricpressure of the medium. Since each row 120 of laser writing is triggeredwhen the target 103 has moved one pixel or grid unit, a realtime streamof position pulses is required having a periodicity equal to the pixelor grid unit spacing. The conversion from wavelength units to pixelunits or grid units is done by the autocompensator 31. This same form ofcompensation is applied when patterns are written that have a slightlydifferent size than the reference database. This is important forwriting sequentially built printed wiring boards, or any alignedmulti-layer structure. This same dimensional compensation also isrequired for pattern inspection to achieve registration between thereference database and the target pattern being inspected. In such acase, the correction is based upon alignment marks included on thetarget 103. The autocompensator 31 scale factor is calculated to adjustthe row scan units to coincide exactly with the alignment marks spacing.

The compensation is achieved by programmed pulse rate scaling. Oneimplementation of programmed pulse rate scaling is by repeated additionor subtraction. In one form, this is done using an arithmeticaccumulator having as many bits as the finest increment desired in thecorrection, i.e., if correction resolution of one part in 1024 isdesired, then a ten bit adder is required. For each incoming positionpulse, a constant is added to the accumulator for each positivedirection pulse and the complement of the constant is added for eachpulse in the negative direction. Overflow (carry out of the accumulator)is a positive direction output pulse and underflow is a negativedirection output pulse. The constant to be added is calculated bydividing the number of output pulses by the number of input pulses, andmultiplying the result by the maximum value that can be stored in theaccumulator. The number of output pulses per input pulse cannot exceedone. This method produces smoother output if the units are scaled sothat there are several input pulses for each output pulse.

The autocompensator 31 also performs fixed scale division on stage 118position and direction of travel pulses from both the X and Y directionsand performs a programmable pulse output rate scaling operation tocompensate for environmental variations.

The autocompensator 31 receives X and Y position pulses corresponding toa one micron distance of stage 118 travel per pulse. The autocompensatortransmits compensated 5 or 10 micron X and Y position pulses to thestage controller 25. That is, the autocompensator transmits one pulsefor every 5 or 10 pulses received from the X and Y axis blocks 34 and33. Whether 5 or 10 pulse resolution is used depends upon whether 5micron or 10 micron spacing between pixel centers 128 is desired. Thus,the autocompensator scales the finer resolution X and Y position pulsesoriginating with the glass scale encoders or the laser interferometers,to a resolution which is not so fine but is resolved in units of pixels.

Smoothing Filter

Accurate laser writing of individual pixels depends on accuratelyspacing rows 120 of pixels 119 in the Y direction, and accuratelybeginning the writing of each row 120 of pixels. This requires accurateinformation as to the X and Y position of the stage 118. Although glassscale encoders may be desirable for a particular application due totheir speed, they can suffer from local inaccuracies in the sine wavethat they output to the pulse generating electronics in blocks 34 and33. These inaccuracies result in inaccurate time spacing between some ofthe pulses. Some of these inaccuracies result from artifacts in thegrating uniformity on the glass scales of the glass scale encoders.These inaccuracies can result in unstable phase relationships betweenthe quadrature outputs of the grating sensors. This can result in shortrange shifts in the indicated pulse position, which causes a short rangeshift in the X or Y position calculated by the stage controller 25. Witha well made grating, cumulative or long term errors are very small ornonexistent. The short range error is reduced in severity by thesmoothing filter 30.

The smoothing filter 30 receives glass scale encoder generated X and Yposition pulses and stage 118 travel direction information from theautocompensator 31. The position pulses received from theautocompensator are sporadic and not evenly spaced apart. This can beseen by referring to FIG. 7, which shows the actual pulse train 141 foreither X or Y input by the smoothing filter 30 from the autocompensator31. The actual pulse train 141 is shown for purposes of illustration andcan represent either the X or Y position information. For purposes ofthis illustration, the stage 118 is assumed to be moving at constantspeed, which it does most of the time with a high degree on consistencydue to the inertia from the large granite mass of the stage. The idealpulse train 140 represents what the pulse train from the autocompensatorshould look like when the stage is travelling at a constant speed pulses137, 138 and 139 of the actual pulse train are shifted in time fromwhere they should be if they are to represent accurately the trueposition of the stage 118. The smoothing filter 30 transforms the pulsetrain 140 representing the movement of a non-accelerating ornon-decelerating stage into an output pulse train having constantspacing between pulses. The pulse train output from the smoothing filter30 will have a constant pulse separation which is nearly the same as theseparation of the ideal pulses.

The smoothing filter 30 averages the time lapsed between the outputpulses of the autocompensator over a 16 pulse window and then outputs atrain of pulses having a constant separation equal to the averageseparation of the pulses from the autocompensator 31. The smoothingfilter 30 outputs this pulse train to the stage controller 25. The 16pulse window is simply the last 16 pulses input by the smoothing filter30 from the autocompensator 31. The window is active when the last 16pulses are being averaged and output to the stage controller. The windowis inactive while the stage is accelerating or decelerating and thepulse train output from the autocompensator simply is passed through tothe stage controller 25 by the smoothing filter 30 without modification.The window is inactive during acceleration or deceleration since theseparation between pulses is undergoing rapid changes and an averagewould give an untrue picture of the stage 118 position. That is, thepulse separation is not expected to remain constant for the last 16pulses. All input pulse separation averaging logic will be reset if thestage changes its direction of travel.

The smoothing filter 30 includes a bit-slice arithmetic logic unit and apulse interval counter. FIG. 8 illustrates the flow of functions of thesmoothing filter. As indicated at block 142, the pulse interval counteroperates in a free-running mode to calculate the time lapse betweeninput pulses. The smoothing filter 30 includes a pulse intervalaccumulator (memory) which stores the total amount of time transpiredover the most recent 16 input pulses, as indicated at block 143. Asindicated at block 144, the smoothing filter uses the total time savedin the accumulator to compute the average time lapse between the inputpulses received from the autocompensator 31. The average time lapse isused as the spacing for the pulses output to the stage whether thewindow is active. If the window is active, then the pulses output fromthe smoothing filter will have a frequency determined by the averagepulse spacing, as indicated at block 146. If the window is inactive,then the smoothing filter passes the input pulses through to the stagecontroller without change, as seen at block 147.

The pulse interval counter is a free running counter that counts thenumber of clock cycles occurring between pulses.

The pulse interval accumulator holds the total amount of time transpiredover the most recent 16 input pulses. As each new pulse arrives, thevalue found in the pulse interval counter is added to the accumulatorand stored. This continues over the first 16 pulses. Beyond the first 16pulses, the oldest value is subtracted from the accumulator before thenew value is added.

The pulse interval averager computes the average time lapse betweeninput pulses to determine the output pulse spacing. Prior to receptionof the first 16 input pulses, when the window is closed, the pulseinterval averager values are not used, and input pulses are passeddirectly to the stage controller 25. Upon reception of the sixteenthinput pulse and all pulses thereafter, the accumulator is divided by 16.This value is continually updated upon each pulse received for theremainder of the window opening or until errors occur.

Pattern Writer Data Flow

The database on tape 17, shown in FIGS. 6 and 9, for a part which is tobe written on photosensitive surface 121, is identical to the databaseon tape 17 for a part which is to be inspected. Therefore, the followingdescription of the organization of the database as it exists on tape 17is applicable equally to a pattern writer system 50 or a patterninspection system 400.

Tape 17 is organized into a series of records, each 4000 bytes long. Allinformation stored on the tape is stored in 16 bit words, and allnumeric values will be limited to 32767.

The database stored on tape 17 is comprised of two Main DirectoryRecords, followed by two Target 103 Header Records, which in turn arefollowed by however many Data Records as are required to describe thegeometrical patterns to be written on photosensitive surface 121.

FIG. 10 is a table indicating the contents of Main Directory Record 1.As is evident from the table, this record contains information usefulfor documentation purposes, such as the date the tape was generated.Record 1 also contains information necessary for the correct scaling ofthe database to real-world dimensions. For example, Word 5 is a numberin binary which is in 0.01 micron units. Typically for printed wiringboard writing, this number will be 100 decimal, that is, the unit sizeis one micron. This one micron value is the size of one address unit.One address unit is also the size of one pixel.

Data records stored on tape 17 are organized so that a target 103surface 121, as shown in FIGS. 1-4, is treated as though it issubdivided into strips 105 parallel to the Y axis, each of which is 1024pixels 119 in width. Each strip 105 is subdivided into frames 106 eachof which is 512 pixels high. Thus, the entire target surface 121 willconsist of rectangles 1024 by 512 pixels in size. The strips 105 startat the left side of the target 103 and in the data records are numberedfrom zero to N, where (N+1)*1024 is greater than or equal to the targetsurface 121 width. The frames 106 for each strip start at the top ofthat strip and in the data records are numbered from zero to K, where(K+1)*512 is greater than or equal to the target surface 121 height.

Returning to FIG. 10, Word 7 indicates the strip 105 (frame 106) widthin address units (pixels). Typically this value will be 1024. Word 8 isthe frame 106 height in address units. Typically this value will be 512.

Word 9 indicates whether writing or inspection will take place. The typeof target is also indicated, e.g., a reticle or mask. Reticles and maskshave been mentioned by way of example only, and it is understood thatprinted wiring boards, semiconductor wafers and other targets can alsobe written on or inspected.

Word 53 indicates whether a target 103 is to be written on or inspectedin serpentine fashion or from the top to the bottom of every strip 105.

FIG. 11 is a table indicating the contents of Main Directory Record 2.This record mainly keeps track of the Header Record number for each setof target data records stored on the tape 17. It is apparent that a tapemay contain data records for writing or inspecting more than one targetpattern. By first target, second target and so on, sets of target datarecords are being indicated. Each set of target data records defines ageometrical pattern which is to be written on a target 103 or it definesan ideal pattern against which an actual physical pattern on a target103 can be compared.

For each set of target data records there are two Target Header Records,although Target Header Record 2 is not presently used. FIG. 12 is atable indicating the contents of Target serve as a standard forinspection, is constructed of polygons which do not cross frame 106boundaries. Even patterns with curved lines can accurately berepresented by polygons with straight sides if the polygons aresufficiently small. Words 3 and 4 indicate the number of these polygonsfor that set of target records.

Word 5 indicates the number of the first strip that contains non-zerodata. That is, the first strip containing any polygons. Likewise, Word 6indicates the number of the last strip 105 that contains non-zero data.Word 7 indicates the number of the first frame 106 that containsnon-zero data, and Word 8 indicates the number of the last frame thatcontains non-zero data. Thus, the stage 118 can be immediately advancedto the first strip 105 and frame 106 that contain non-zero data to beginraster scanning the target 103 surface 121 at that frame. Similarly,raster scanning can be made to cease as soon as the last strip and framecontaining non-zero data have been raster scanned. This can result inquicker run-time of the writing or inspection processes.

The geometric data on the tape is contained in the Data Recordsfollowing the Target Header Records. This data describes one or morepolygons which, when taken together, forms an approximation of thegeometrical pattern which is to be written on target surface 121, orwhich is on target surface 121 and will be inspected and comparedagainst the ideal pattern defined by the data records. Each polygon iscompletely contained within a frame 106. Furthermore, each polygon mustbe a convex polygon in the X direction. That is, for a polygon to beconvex in the X direction, any line drawn parallel to the X axis mustintersect the boundary of the polygon twice at most. This is requiredbecause polygons are written or inspected by raster scanning rowsparallel to the X axis. Each polygon is written by turning the laserbeam on when the left edge of the polygon is encountered and turning thelaser beam off when the right edge of the polygon is reached. That is,all the pixels 119 within the polygon must be written to or inspected.If polygons were not convex in the X direction, the laser beam wouldneed to be turned on more than once and off more than once for the samepolygon.

For example, FIG. 13 shows a target 103 consisting of two strips 105 andeight frames 106. It is desired to write, or inspect, a filled circle200 (a disk) on surface 121 so that the filled circle overlaps frames201, 202, 203 and 204. The filled circle 200 is represented in the datarecords as being made up of eight polygons. During writing process, thelaser writer system 50 will write on surface 121 the geometrical shapedefined by the eight contiguous polygons. During a process of inspectinga pre-existing pattern on surface 121, such as a developed pattern on aphotoresist covered board, the pre-existing pattern is compared againstthe ideal pattern defined by the eight polygons.

Each polygon lies completely within a single frame. For example,polygons 205 and 207 both lie completely in frame 201; polygon 208 liescompletely in frame 203; and polygon 206 lies completely in frame 202.Polygons 205 and 206 are contiguous With a common border along stripboundary 209. Polygons 207 and 208 are contiguous with a common borderalong frame boundary 210. The geometrical pattern defined by thepolygons will more closely approximate the filled circle 200 if morethan eight polygons are used to describe the filled circle. This is alimit process and is similar to some numerical methods of integration,or approximations of the value of an integral. As many polygons are usedto describe a geometrical pattern as are necessary for predeterminedtolerances. For example, the tolerances allowed on a printed wiringboard will become tighter as the circuit density of the board increases.Likewise, more accuracy is required for multi-layer artwork thantypically would be necessary for single layer artwork.

The geometric data in the Data Records is subdivided into contiguousframes of data which correspond to the frames 106 shown superimposed onthe target surface 121. All frames are indicated or represented in theData Records even if there is no polygon data in the frame.Corresponding to each superimposed frame 106 containing one or morepolygons, is a frame of data containing data descriptive of thosepolygons. Each such polygon is described by the X-Y coordinate of on ofthe vertices of the polygon and various X and Y displacements. All X-Ycoordinates are positive displacements from the upper left corner of theframe, which is considered to be the origin for that frame. The exactform of the description depends upon the geometrical properties of thepolygon. There are nine categories of polygons which can be used in theData Records. Some of these categories are further subdivided into twoor four subcategories.

Each polygon's descriptive data is immediately preceded by anidentifying 16 bit control word, illustrated in the table in FIG. 23.Bits zero through five of this control word are used to designate thecategory and subcategory (if applicable) of polygon whose dataimmediately follows this control word.

Bits 8 through 11 of the identifying control word of FIG. 23, containinformation, concerning the polygon in the description following theidentifying control word, which is useful in generating guardbands(don't-care zones) around the polygon during the inspection process.Specifically, bits 8 through 11 indicate whether the polygon beingdescribed has a side(s) on the left, right, top or bottom frame boundaryand is in common with another polygon in the next frame to the left,right, top or bottom, respectively. If two polygons have a frameboundary, or portion thereof, in common, then, those two polygonstogether describe a larger, inclusive polygon on whose sides andinterior all pixels are part of the pattern to be written during writingor compared with during inspection. Guardbands are generated by DPC2 15shown in FIGS. 6 and 9.

The database as it exists on tape 17 is the same for a writer and aninspector. During the writing process, no guardbands are generated andthe information in bits eight through eleven is ignored by the writersystem 50.

FIG. 14 illustrates the category of polygons which is comprised ofrectangles. Rectangle R is shown contained within frame 106, and isdescribed in the Data Records by four 16 bit words following theidentifying control word. The first word is the X coordinate and thesecond word the Y coordinate of lower left corner 213. The third word isthe width 211 in pixels 119, and the fourth word is the height 212 inpixels.

Bits 0 through 5 of the identifying control word equals 07 inhexadecimal which indicates that the data words following the controlword describe a rectangle. Bits 8 through 11 of the indentifying controlword have the following meanings: If bit 8 is a 1, then the bottom edgeof the rectangle R is on the frame 106 boundary and is in common withanother polygon in the next frame below. If bit 9 is 1 the right edge ofthe rectangle R is on the frame boundary and is in common with anotherpolygon in the next frame to the right. If bit 10 is 1 the top edge ofthe rectangle R is on the frame 106 boundary and is in common withanother polygon in the next frame above. If bit 11 is equal to 1 thenthe left edge of the rectangle R is on the frame boundary and is incommon with another polygon in the next frame to the left.

FIG. 15 illustrates the category of polygons which is comprised oftrapezoids T1 having horizontal top and bottom sides and a vertical leftside. Although the trapezoid T1 which is has opposite sign relative toconventional cartesian coordinate system), this category of polygonsincludes negative slopes as well. Following the identifying control wordwill be either 4 or 5 words depending upon whether or not the slope ofthe right side is or -1. The right side has slope equal to 1 if bits 0through 5 equal 08 hex, or slope -1 if bits 0 through 5 equal 09 hex,provided bit 12 is 1. If the slope is + or -1 then only four data wordsare required to describe the polygon.

This trapezoid T1 of FIG. 15 is described by the following five words:Word 1 is the X coordinate and word 2 is the Y coordinate of the lowerleft corner 217. Word 3 is height 214 and word 4 is width 216. Word 5 isthe width 215. If only four words are required to describe the trapezoidT1, then word 5 which is the width 215 is not used.

Bits 0 through 5 of the identifying control word equal 08 hex if theright side has a positive slope, and equal 09 hex if the right side hasa negative slope. If bit 8 is 1 the bottom edge of the trapezoid T1 ison the frame boundary and is in common with another polygon in the nextframe 106 below. Bit 9 is 0. If Bit 10 is 1 then the top edge of thetrapezoid T1 is on the frame boundary and is in common with anotherpolygon in the previous frame above. If bit 11 is 1 then the left edgeof the trapezoid T1 is on the frame boundary and is in common withanother polygon in the next frame to the left. If bit 12 is 1 then theright side of the trapezoid T1 is at 45 degrees and hence width 215 isnot used. Bit 13 will be 0.

FIG. 16 illustrates the category of polygons which is comprised oftrapezoids T2 having horizontal top and bottom sides, a vertical rightside and a sloping left side. Although the trapezoid T2 is shown with aleft side having a negative slope, this category of polygons includespolygons with positive slopes as well. Following the identifying controlword will be either 4 or 5 words describing the trapezoid, dependingupon whether or not the slope of the left side is + or -1. The left sidehas slope equal to 1 if bits 0 through 5 equal 0A hex, or slope -1 ifbits 0 through 5 equal 0B hex, provided bit 13 is 1. If the slope is +or -1 then only four data words are required to describe the polygon.

If the slope of the left side of trapezoid T2 is not + or -1, then thefollowing five words are required to describe the polygon: Word 1 is theX coordinate and word 2 is the Y coordinate of the lower left corner247. Word 3 is height 220 and word 4 is width 218. Word 5 is the width219. If only four words are required to describe the trapezoid T2, thenword 5, which is width 219, is not used.

Bits 0 through 5 of the identifying control word equal 0A hex if theleft side has a positive slope, and equal 0B hex if the left side has anegative slope. If bit 8 is a 1 then the bottom edge is on the frame 106boundary and is in common with another polygon in the next frame below.If bit 9 is a 1, then the right edge is on the frame boundary and is incommon with another polygon in the next frame to the right. If bit 10 isa 1, then the top edge if the polygon is on the frame boundary and is incommon with another polygon in the previous frame above. Bits 11 and 12are 0. If bit 13 is 1 then the left side has a 45 degree slope and width219 is not used to describe the polygon T2.

FIG. 17 illustrates the category of polygons which is comprised ofparallelograms P1 having horizontal top and bottom sides, and the othertwo sides are sloping. Parallelogram P1 is shown with negative slopingsides, however, this category of polygons includes parallelograms withpositive slopes as well. Following the identifying control word will beeither 4 or 5 words describing the parallelogram P1, depending uponwhether or not the slope of the slanted sides is + or -1. The slantedsides have a slope of +1 if bits 0 through 5 equal 0C hex, or slope -1if bits 0 through 5 equal 0C hex, provided bits 21 and 13 both are 1. Ifthe slope is + or -1 then only four data words are required to describethe polygon P1.

If the slope of the slanted sides of parallelogram P1 is not + or -1,then the following five words are required to describe the polygon: Word1 is the X coordinate and word 2 is the Y coordinate of the lower leftvertex 224. Word 3 is height 222 and word 4 is width 221. Word 5 is delX223. If only four words are required to describe the parallelogram P1,then the word 5, which is delX 223, is not used.

Bits 0 through 5 of the identifying control word equal 0C hex if theslanted sides have a positive slope, and equal 0D hex if the slantedsides have a negative slope. If bit 8 is 1 then the bottom edge is onthe frame 106 boundary and is in common with another polygon in the nextframe below. Bit 9 is 0. If Bit 10 is 1 then the top edge of theparallelogram is on the frame boundary and is in common with anotherpolygon in the previous frame above. Bit 11 is 0. If bits 12 and 13 areboth -1, then the slanted sides are at 45 degrees and delX 223 is notused.

FIG. 18 illustrates the category of polygons which is comprised oftrapezoids T3 having horizontal top and bottom sides, and the left andright sides are slanted. Although the trapezoid T3 is shown with a leftside having a positive slope and a right side having a negative slope,this category of polygons includes polygons having left and right sideswith either positive or negative slopes in any combination. Followingthe identifying control word will be either 4, 5 or 6 words describingthe trapezoid T3, depending upon whether or not the slopes of the leftand/or right sides are + or -1. The left and right sides have slopesequal to 1 if bits 0 through 5 equal 0E hex, or slopes equal to -1 ifbits 0 through 5 equal 0F hex, provided bits 12 and 13 are both 1. Thusonly four data words are required to describe the polygon. Likewise, theleft side has slope equal to one and the right side has slope equal to-1, provided bits 0 through 5 equal 10 hex and bits 12 and 13 areboth 1. Thus, only four data words are required to describe the polygon.Similarly, the left side has slope equal to -1 and the right side hasslope equal to 1, provided bits 0 through 5 equal 11 hex and bits 12 and13 are both 1. In this case as well, only four data words are requiredto describe the polygon.

If four data words only are required to describe the polygon, the wordsare the following: Word 1 is the X coordinate and word 2 is the Ycoordinate of the lower left vertex 229. Word 3 is height 228 and word 4is width 225.

If either the left side or the right side (but not both) of polygon T3is equal to + or -1, then five words will be required to describepolygon T3. In this case, the same four words are required to describethe polygon T3 as above, but in addition, either delX 226 or delX 227will follow width 225. DelX 226 is required if the left side does nothave a slope of + or -1. DelX 227 is required if the right side does nothave a slope of + or 1.

If neither the right or left sides have slopes of + or -1, then sixwords must be used to describe the polygon T3. That is, both delX 226and delX 227 are required. DelX 226 followed by delX 227 will follow thewidth 225.

Bits 0 through 5 of the identifying control word equal 0E hex if boththe left and the right sides have positive slope, equal 0F hex if boththe left and right sides have negative slope, equal 10 hex if the leftside has positive slope and the right side has negative slope, and equal11 hex if the left side has negative slope and the right side haspositive slope. If bit 8 is 1, then the bottom edge of the trapezoid T3is on the frame 106 boundary and is in common with another polygon inthe next frame below. Bit 9 is 0. If bit 10 is 1, then the top edge ofthe trapezoid T3 is on the frame boundary and is in common with anotherpolygon in the previous frame above. Bit 11 will be off. If bit 12 is onthen the right slantedside is 45 degrees and hence delX 227 is not used.If bit 13 is 1, then the left slanted side is at 45 degrees and hencedelX 226 is not used in the polygon description.

FIG. 19 illustrates the category of polygons which is comprised oftrapezoids T4 having left and right vertical sides, a horizontal topside and a slanted bottom side. Although the trapezoid T4 is shown witha bottom side having a positive slope, this category of polygonsincludes polygons with negative slopes as well. Following theidentifying control word will be either 4 or 5 words describing thetrapezoid T4, depending upon whether or not the slope of the bottom sideis + or -1. The bottom side has slope equal to +1 if bits 0 through 5equal 12 hex, or slope equal to -1 if bits 0 through 5 equal 13 hex,provided bit 13 is 1. If the slope is + or -1 then only four data wordsare required to describe the trapezoid T4.

If the slope of the bottom side of trapezoid T4 is not + or 1, then thefollowing five words are required to describe the polygon: Word 1 is theX coordinate and word 2 is the Y coordinate of the lower right vertex233. Word 3 is width 230 and word 4 is height 231. Word 5 is the height232. If only four words are required to describe the trapezoid T4, thenword 5, which is height 232, is not used.

Bits 0 through 5 of the identifying control word equal 12 hex if thebottom side has a positive slope, and equal 13 hex if the bottom sidehas a negative slope. Bit 8 is 0. If bit 9 is 1, then the right edge ofthe trapezoid is on the frame 106 boundary and is in common with anotherpolygon in the next frame to the right. If bit 10 is 1, then the topedge of the polygon T4 is on the frame boundary and is in common withanother polygon in the previous frame above. If bit 11 is 1, then theleft edge of the trapezoid T4 is on the frame boundary and is in commonwith another polygon in the next frame to the left. Bit 12 is 0. If bit13 is 1, then the slanted bottom side is at 45 degrees and hence theheight 232 is not used.

FIG. 20 illustrates the category of polygons which is comprised oftrapezoids T5 having left and right vertical sides, a horizontal bottomside and a slanted top side. Although the trapezoid T5 is shown with atop side having a negative slope, this category of polygons includespolygons with positive slopes as well. Following the identifying controlword will be either 4 or 5 words describing the trapezoid, dependingupon whether or not the slope of the top side is + or -1. The top sidehas slope equal to 1 if bits 0 through 5, of the identifying controlword, equal 14 hex, or slope -1 if bits 0 through 5 equal 15 hex,provided bit 12 of the identifying control word is 1. If the slope is +or -1 then only four data words are required to describe the polygon.

If the slope of the top side of trapezoid T5 is not + or -1, then thefollowing five words are required to describe the polygon: Word 1 is theX coordinate and word 2 is the Y coordinate of the lower right corner237. Word 3 width 234 and word 4 is height 235. Word 5 is the height236. If only four words are required to describe the trapezoid T5, thenword 5, which is height 236, is not used.

Bits 0 through 5 of the identifying control word equal 14 hex if the topside of trapezoid T5 has a positive slope, and equal 15 hex if the topside has a negative slope. If bit 8 is 1, then the bottom edge oftrapezoid T5 is on the frame 106 boundary and is in common with anotherpolygon in the next frame below. If bit 9 is 1, then the right edge oftrapezoid T5 is on the frame boundary and is in common with anotherpolygon in the next frame to the right. Bit 10 is 0. If bit 11 is 1,then the left edge of trapezoid T5 is on the frame boundary and is incommon with another polygon in the next frame to the left. If bit 12 is1, then the slanted top side of trapezoid T5 has slope of 45 degrees,hence height 236 is not used. Bit 13 is 0.

FIG. 21 illustrates the category of polygons which is comprised ofparallelograms P2 having left and right vertical sides, and slated topand bottom sides. Although the parallelogram P2 is shown with slantedsides having a negative slope, this category of polygons includesparallelograms with positive sloped top and bottom sides as well.Following the identifying control word will be either 4 or 5 wordsdescribing the parallelogram, depending upon whether or not the slope ofthe top and bottom sides is + or -1. The top and bottom sides have aslope equal to 1 if bits 0 through 5 of the identifying control wordequal 16 hex, or slope -1 if bits 0 through 5 equal 17 hex, providedbits the slope is + or -1 then only four data words are required todescribe the parallelogram.

If the slope of the top and bottom sides of the parallelogram P2 isnot + or -1, then the following five words are required to describe theparallelogram: Word 1 is the X coordinate and word 2 is the Y coordinateof the lower right vertex 241. Word 3 is width 238 and word 4 is height239. Word 5 is delY 240. If only four words are required to describe theparallelogram P2, then word 5, which is dely 240, is not used.

Bits 0 through 5 of the identifying control word equal 16 hex if the topand bottom sides have a positive slope, and equal 17 hex if the top andbottom sides have a negative slope. Bit 8 is 0. If bit 9 is 1, then theright edge of the parallelogram P2 is on the frame boundary and is incommon with another polygon in the next frame to the right. Bit 10 is 0.If bit 11 is 1, then the left edge of parallelogram P2 is on the frame106 boundary and is in common with another polygon in the next frame tothe left. If bits 12 and 13 or both 1 then the top and bottom sides haveslope equal to 45 degrees, hence delY 240 is not used.

FIG. 22 illustrates the category of polygons which is comprised oftrapezoids T6 having vertical left and right sides, and slanted top andbottom sides. Although the trapezoid T6 is shown with the top sidehaving a negative slope and the bottom side having a positive slope,this category of polygons also includes polygons with top and bottomsides having any combination of positive and negative slopes. Followingthe identifying control word will be either 4, 5 or 6 words describingthe trapezoid T6, depending upon whether or not the slopes of the topand bottom sides are + or -1. The top side has slope equal to +1 if bits0 through 5 of the identifying control word equal 18 hex or 1B hex, orslope equal to -1 if bits 0 through 5 equal 19 hex or 1A hex, providedthat bit 12 is equal to 1. The bottom side has slope equal to +1 if bits0 through 5 of the identifying control word equal 18 hex or 1A hex, orslope equal to -1 if bits 0 through 5 equal 19 hex or 1B hex, providedthat bit 13 is equal to 1.

If both top and bottom sides have a slope equal to + or -1, then onlyfour words are required to describe the trapezoid T6. Those four wordsare: Word 1 is the X coordinate and word 2 is the Y coordinate of thelower right vertex 246. Word 3 is width 243 and word 4 is height 242.

If the top side has a slope equal to + or -1, and the bottom side has aslope not equal to + or -1, then, in addition to words 1 through 4, word5 is also required to describe trapezoid T6. Word 5 is delY 244.

If the bottom side has a slope equal to + or -1, and the top side has aslope not equal to + or -1, then, in addition to words 1 through 4, word5 is also required to describe trapezoid T6. Word 5 is delY 245. Word 5is now defined differently than in the case described in the precedingparagraph.

If neither top or bottom sides have slope equal to + or -1, then sixwords are required to describe the trapezoid T6. Words 1 through 4 arethe same as in the other three cases. Word 5 now is delY 244 and word 6is delY 245.

Bits 0 through 5 of the identifying control word equal 18 hex when bothtop and bottom sides have positive slopes. Bits 0 through 5 equal 19 hexwhen both top and bottom sides have negative slopes. Bits 0 through 5equal 1A hex when the bottom side has positive slope and the top sidehas negative slope. Bits 0 through 5 equal 1B hex when the bottom sidehas negative slope and the top side has positive slope.

Bit 8 of the identifying control word equals 0. If bit 9 is 1, then theright side of trapezoid T6 is on the frame 106 boundary and is in commonwith another polygon in the next frame to the right. Bit 10 is 0. If bit11 is 1, then the left side of the trapezoid T6 is on the frame boundaryand is in common with another polygon in the next frame to the left. Ifbit 12 is 1, then the top side has 45 degree slope, hence delY 244 isnot used to describe trapezoid T6. If bit 13 is 1, then the bottom sidehas a 45 degree slope, hence delY 245 is not used to describe trapezoidT6.

FIGS. 23 and 24 are tables indicating the meanings of the bitsconstituting the 16 bit control words which are embedded throughout theData Records of the. database. A one in bit position 15 indicates that16 bit word is a control word. The type of control word it is indicatedby bits 0 through 5. These bits are called the control bits. Forinstance, the meanings of the control bits has been explained inconnection with FIGS. 14-22 when their value is 07 hex through 1B hex.When bits 0 through 5 take on one of the values 07 hex through 1B hex,the control word is referred to as an identifying control word. Theidentifying control word identifies the characteristics of the polygonrepresented by the data words immediately following the control word inthe Data Records.

FIG. 24 lists the control bits and their meaning for seven types ofcontrol words other than identifying control words. When the controlbits equal 00 hex, that control word is the first word to appear on thetape and it precedes the first Main Directory File. The other controlwords can be understood in the outline of the Data Records shown in FIG.25.

The first word shown in FIG. 25 is the Beginning of Strip control word.Each strip of data begins with this word. This control word is followedby a 16 bit strip ID number. Next follows the Beginning of Frame controlword, followed by a 16 bit frame ID number. This is followed by theidentifying control word, Beginning of Polygon. The data defining thepolygon follows. After this, another polygon is indicated and definedfollowed by control words indicating the End of Frame, Frame ID, End ofStrip and then End of Target. End of Target indicates that there is nomore data to be written on target 103 surface 121 or, alternatively,that there is no more ideal pattern information against which an actualpattern is to be compared. In the example of FIG. 24, two polygons werein one frame, in a single strip, however, many polygons can be in oneframe, and there may be many frames and strips.

A Pattern Writing Example

FIG. 9 illustrates the data flow for a pattern writer system. This flowwill be illustrated by examining the data flow to write a rectangle 300,shown in FIG. 26, onto the photosensitive surface 121 of target 103.Rectangle 300 straddles upper frame 301 and lower frame 302 with equalamounts in each frame. The writing process will illuminate each pixel119 on and in the interior of rectangle 300. The portion of therectangle 300 in the upper frame 301 is the rectangle 311 defined byvertices 303 at (100,100), 304 at (200,100), 305 at (100,1FF), and 306at (200,1FF). The portion of the rectangle 300 in the lower frame 302 isthe rectangle 312 defined by vertices 307 at (100,0), 308 at 200,0), 309at (100,FF), and 310 at (200,FF). The (X,Y) coordinates are given inhexadecimal. The pixel at 305 is adjacent the pixel at 307, and thepixel at 306 is adjacent the pixel at 308. The database 313 forrectangle 300 is divided between frames 301 and 302. The first frame ofthe database contains the data defining rectangle (polygon) 311, whilethe second frame contains the data defining rectangle (polygon) 312.

The flow of data for the pattern writing process begins when DPC2 15reads the database for rectangle 300 from tape 17, transforms it intoturnpoint polygon representation, and transfers it onto data disk 13.FIG. 27 shows the tape 17 resident database 313 in hexadecimal forrectangle 300. This data 313 begins with the control word for beginningof strip, followed by the strip ID. The rectangle 300 is in the firststrip 105, so that the strip ID is zero.

Following the strip ID is the data 314 for the rectangle 311 which isthe polygon in upper frame 301. Data 314 begins with the control wordfor beginning of frame, which is followed by the frame ID. Upper frame301 is the first frame in the strip so the frame ID is zero. The nextdata word is the beginning of rectangle (polygon) control word 8107 hex.Bits 0-5 equals 07 hex for rectangle. Bit 8 equals 1 to indicate thatthe bottom side of the rectangle 311 is on the frame boundary 318, andis in common with the rectangle 312 in the next frame, frame 302. Thebeginning of rectangle control word is followed by the X coordinate andthen the Y coordinate of vertex 305. The Y coordinate is followed by thedatabase rectangle width which is defined to be the actual width 316minus one pixel. Actual width is 101 hex, so that the database rectanglewidth is 100 hex. The database rectangle height OFF hex is defined to beone pixel less than the actual height 317, which is 100 hex. Thedatabase rectangle height is followed by the end of frame control word,which is followed by the frame ID, which is zero for the first frame.

The data 315 for the rectangle 312 in the second frame 302, is similarto the data for the rectangle 311 in the first frame 301. The beginningof rectangle control word 8407 hex has bit 10 equal to 1 to indicatethat the top side of the rectangle 312 is on the frame boundary 318 andis in common with the rectangle 311 in the previous frame, frame 301.

The data 315 for rectangle 312 is followed by the end of strip controlword, which is followed by the end of target control word.

Data Disc 13 Resident Database

DPC 2 15 transforms the tape 17 resident database into a turnpointpolygon representation and sends it to data disk 13, as shown in FIG. 9.FIGS. 29 and 30 are listings of the data disk 13 resident database forrectangle 300. FIG. 28 is a table listing the control words inserted byDPC2 into the database during transformation of the tape 17 residentdatabase into the disk 13 resident database.

The turnpoint polyqon representation of the database on data disk 13consists of 16 bit words of two types, control words, listed in FIG. 28,and data words. As seen in FIG. 28, if bit 15 of a database word equals1, this indicates that word is a control word. Bit 15 equal to 0indicates a data word. Bit 14 is used in the Polygon Start Indicatorcontrol words, to designate that the following polygon data is referencedata if bit 14 equals 1, or is guard band data if bit 14 equals 0.

The Beginning of Target control word occurs once at the beginning of thetarget database. This control word is followed by a 16 bit wordidentifying the target.

The Beginning of Strip control word occurs at the beginning of eachgroup of 16 bit words describing a strip 105. This control word isfollowed by a data word which is the strip number. Strips are numberedfrom left to right on a target surface 121 beginning with zero.

The Beginning of Frame control word occurs at the beginning of eachgroup of 16 bit words describing the polygons within a single frame 106.This control word is followed by a 16 bit data word identifying theframe by number. Frames are numbered from top to bottom within a singlestrip 105, beginning with zero.

The Polygon Start Indicator control word occurs at the beginning of eachgroup of 16 bit data words defining a convex, in the X direction,polygon. In the present example, that polygon is a rectangle 311 or 312.

The Polygon Start Indicator control word is followed by the X-Ycoordinate pairs of each turnpoint of the polygon. A polygon turn pointis a vertex of the polygon, that is, it is the point at which two sidesof the polygon meet at a non-zero, non-180 degree angle. For each pairof X-Y coordinates, the 16 bit X coordinate is followed by the 16 bit Ycoordinate. There is a polygon X-Y coordinate pair for each turnpoint ofthe polygon, and the pairs appear in counter-clockwise order, beginningwith the same X-Y pair that appeared in the tape 17 databaserepresentation of the polygon. Convex, in the X direction, polygonsdefined within a frame may be reference polygons which define the idealpolygons to be written on surface 121 or compared with an actual patternexisting on surface 121. These polygons may also be guard band polygons.Bit 14 of the Polygon Start Indicator control word equals 1 if thepolygon is a reference polygon, or equals 0 if the polygon is a guardband polygon.

The last pair of turnpoint X-Y coordinates is followed by the End ofPolygon control word. There may be more than one polygon in the sameframe, and each polygon's group of 16 bit data words begins with aPolygon Start Indicator control word and ends with an End of Polygoncontrol word.

Following the End of Polygon control word for the last polygon in aframe 106, is the End of Frame control word. There may be, and usuallyare, more than one frame 106 in a strip 105. Each frame begins with aBeginning of Frame control word and ends with an End of Frame controlword.

Following the End of Frame control word for the last frame 106 in astrip 105, is the End of Strip control word. There may be, and usuallyare, more than one strip 105 for a target 103. Each strip begins with aBeginning of Strip control word and ends with an End of Strip controlword.

Following the End of Strip control word for the last strip 105 of target103, is the End of Target control word. This control word signals theend of the database for the target.

FIG. 29 shows data disk resident database 319 for rectangle 300. Thefirst 16 bit word of the database 319 is the Beginning of Target controlword, followed by the target ID. Next is the Beginning of Strip controlword, which is followed by the strip ID. The strip containing rectangle300 is the first (and only) strip 105, therefore its strip ID is zero.

The strip ID is followed by the group 320 of 16 bit words for the upperframe 301. This group of words begins with the Beginning of Framecontrol word, which is followed by the Frame ID. The frame ID is zerosince this frame 301 is the first frame in the strip. After the frame IDcomes the Polygon Start Indicator. Bit 14 of the Polygon Start Indicatoris 1 to signify that the X-Y coordinate pairs to follow are theturnpoints of a reference polygon, rather than those of a guardbandpolygon. The four X-Y coordinate pairs are the coordinates of theturnpoints 305, 306, 304 and 303, shown in FIG. 26. The four turnpointsappear in counterclockwise order around the rectangle, beginning withturnpoint 305. Turnpoint 305 is the vertex appearing in the tape 17resident database 313. Following the coordinates for the last turnpointof rectangle 311, is the End of Polygon control word, which is followedby the End of Frame control word.

As seen in FIG. 30, following the group 320 of 16 bit words for frame301, is the group 321 of 16 bit words for frame 302. The only differencebetween the two groups of words are the frame ID and the turnpoints.

The disk resident database 314 ends with the End of Target control word,which is preceded by the End of Strip control word.

DPC3 to Preprocessors

As shown in FIG. 9, the data disk 17 resident database 319 is read fromthe disk by DPC3 14, transformed into two groups of 25 bit words 324defining the rectangle 300, and output down Pipeline A 322 and PipelineB 323. The first group 325 of 25 bit words represents frame 301, and issent down Pipeline A 322. The second group 326 of 25 bit wordsrepresents frame 302, and is sent down Pipeline B 323. If there was athird frame, the group of words representing that frame would be sentdown Pipeline A 322 after the group representing the second frame 302had been sent down Pipeline B. The groups of words representingcontiguous frames are never sent down the same pipeline. With anembodiment having two pipelines as in this example, the first group goesdown pipeline A, the second down Pipeline B, the third down Pipeline A,and so on, alternating between pipelines. The group of words for everyother frame goes down the same pipeline. Some embodiments of theinvention have more than two pipelines, for instance, four, in whichcase, the group of words for every fourth frame goes down the samepipeline. In this manner, the burden of data processing downstream ofDPC2 can be distributed and the downstream data processing can becarried out in parallel by as many pipelines as are in the embodiment.This allows for faster real-time pattern writing or inspection.

As shown in FIG. 31, in the group 325 of words representing frame 301,the first 25 bit word in a group is a Move To control word. This controlword signifies that the next two 25 bit words will be the X and Ycoordinates, in that order, for the first polygon turnpoint. In thisexample, the polygon is rectangle 311 and the first turnpoint is vertex305 shown in FIG. 26. The first turnpoint is the same vertex whosecoordinates were given in tape 17 resident database 313. The next 25 bitword is a Draw control word. This control word signifies that the nexttwo 25 bit words will be X and Y coordinates, in that order, of aturnpoint which is not the first turnpoint. The remaining two turnpointsof rectangle 311 follow in the same manner, with their X-Y coordinatepairs each following a Draw control word, as shown in FIG. 31. Asbefore, the turnpoints are ordered in a counterclockwise fashion withrespect to the periphery of rectangle 311, following the first turnpoint305. The last X-Y coordinate pair is followed by an End of Polygoncontrol word, which is, in turn, followed by an End of Frame controlword. The 25 bit words for group 325 are sent one-at-a-time, startingwith the Move To control word, down Pipeline A 322. Turning to FIG. 6,it can be seen that the output bus 52 from DPC3 14 to Pipeline A 322,and the output bus 51 from DPC3 to Pipeline B 323, are each 25 bitswide. This allows a 25 bit word-at-a-time to be output down a pipeline.

The second group 326 of 25 bit words, which represents frame 302, isindicated in FIG. 31 at 326. This group is constructed in similarfashion to the first group 325. The differences being that thecoordinates of the turnpoints represent the rectangle 312 in frame 302,and the last 25 bit word in the group is an End of Strip control word.There is no end of target control word since that information is notneeded downstream in the pipelines of DPC3. DPC3 monitors the data beingread from data disk 13 and acts upon receiving the end of target controlword in the data being read from the disk.

Preprocessor to Figure Filler

The 25 bit words sent down Pipeline A 322 by DPC3 are received byPreprocessor1, indicated at 20A, and the 25 bit words sent down PipelineB 323 by DPC3 are received by Preprocessor 2, indicated at 20B. APreprocessor transforms the turnpoint representation of a polygon,received from DPC3, into one or more left side vectors and one or moreright side vectors. These vectors define the left and right limits ofthe polygon, so that all pixels between or on the vectors will beincluded in a bit map created by the downstream Filler module 23A or23B, of pixels to be illuminated by the laser beam. A pixel isconsidered to be between the left and right vectors if the pixel liesbetween points on the left and right vectors having the same Ycoordinate as the pixel.

FIG. 33 illustrates the left vector 327 right vector 328 representationof rectangle 311, and the left vector 329 right vector 330representation of rectangle 312. In the case of a polygon which is arectangle, the left and right vectors are each single vertical vectors,as shown in FIG. 33. It is understood, however, that there are manypolygons which would require two or more left vectors and/or two or moreright vectors. With a rectangle, the right vector is considered to pointupwards, with the vector origin at the lower right turnpoint. This isshown in FIG. 33 where right vectors 328 and 330 have vector origins atturnpoints 306 and 310, respectively. The left vectors 327 and 329 pointdownwards, with origins at turnpoints 303 and 307, respectively. Thedirection in which the vectors point distinguishes the right vectorsfrom the left vectors.

The Preprocessors 20A and 20B each transform the 25 bit word turnpointpolygon representations from DPC3 14, into 22 bit word left and rightvector representations of the polygons. The general sequence of 22 bitwords used to represent a single polygon is shown in FIG. 34. Words 350through 355 is the general sequence of words used to describe a singlevector.

The slope code NN in word 350 designates which of five possible rangesthe slope of the vector falls. A vector can be horizontal, in which casethere is no need to represent it in the data. If NN is equal to 04, thenthe absolute value of the slope of the vector is greater than one. If NNequals 07, then the vector is vertically oriented. If NN is equal to 08,then the absolute value of the slope of the vector is equal to one. IfNN is equal to 09, then the absolute value of the slope is less thanone. For word 350, the value in the data field 334 is termed the vectorlength. If NN is equal to 07, 08 or 09, then the vector lengthrepresents the Y component of the length of the vector being described.If NN is equal to 04, then the vector length represents the X componentof the length of the vector being described.

If NN in word 350 is equal to 04 or 09, then all the words 350-355 areused to describe the vector.

Word 351 contains only data. If NN of word 350 is equal to 08 or 09,then the data field 334 of word 351 represents the X component of thelength of the vector being described. If NN is equal to 04, then thedata field 334 of word 351 represents te Y component of the length ofthe vector being described.

Word 351 is not used if NN of word 350 is equal to 07. That is, thevector is vertical and there is no X component of its length. This isthe situation illustrated in FIG. 32 in which all left and right vectorsare vertical.

Word 352 contains only data. The data field 334 of this word is the Xcoordinate of the origin of the vector. This word is used for all valuesof NN in word 350.

Word 353 contains only data. The data field 334 of this word is the Ycoordinate of the origin of the vector. This word is used for all valuesof NN in word 350.

Word 354 contains only data. The data field 334 of this word is 0001 ifthe vector points to the right of vertical, and is FFFF if the vectorpoints to the left of vertical. This word is not used when NN equals 07.That is, when the vector is vertical it points neither left nor right,so the word is not needed. This word is used for the other values of NN.

Word 354 is not used if NN of word 350 is equal to 07. That is, thevector is vertical and points neither to the left nor the right.Therefore, there is no need of this word.

Word 355 contains only data. The data field 334 of this word is 0001 ifthe vector points below the horizontal, and is FFFF if the vector pointsabove the horizontal. This word is used for all values of NN in word350.

There are as many sets of the sequence of words 350-355, as there arevectors for an individual polygon. After the last sequence of words350-355 for an individual polygon, comes word 356. Word 356 includes a02 in the command field, which indicates the end of the vector data forthat polygon. The data field 334 of word 356, gives the minimum Y valuefor the polygon. The next word, word 357, contains only data. The datafield 334 of this word gives the maximum Y value for the polygon. Theminimum and maximum Y values for a polygon are used downstream by one ofthe Filler Modules 23A and 23B to speed up the process of bit-mappingthe polygon. The Filler Module can skip right to the first Y value usedby a polygon without having to scan a large amount of data to determinewhere the first polygon in a frame begins. Likewise, the Filler Moduleknows from the maximum Y value when it is through scanning data for aframe, and time is saved.

The sequence of words 350-355 is repeated as many times as there arevectors in an individual polygon, then terminating with words 356 and357. The complete sequence of multiple subsequences of words 350-355,plus the final words 356 and 357, is repeated as many times in a frame106 as there are individual polygons.

FIG. 32 is a listing 331 of the 22 bit words giving left and rightvector descriptions of rectangles 311 and 312. The sequence of 25 bitwords 332 describe rectangle 311 in frame 301, and the sequence of words333 describe rectangle 312 in frame 302.

Sequence 336 and sequence 337 of 22 bit words describing the rightvector 328 and the left vector 327, respectively, each require the useof words 350, 352, 353 and 355, shown in FIG. 34. The first word insequence 336 gives a vector length of FF hex, which is defined as beingone pixel less than the actual length 100 hex. 100 hex is the length ofthe vector as well as being the Y component of the length of the vector,since the right vector 328 is vertical. The following word gives the Xaddress of the origin of vector 328, and the next word gives the Yaddress of the origin. The last word is sequence 336 gives the vectordirection, which signifies that vector 328 is pointing upwards. Sequence337 defines left vector 327 in similar fashion.

Following sequence 337, is the 22 bit word signifying the end of leftand right vector data, and also giving the minimum Y value for rectangle311, of 100 hex. The last word in the sequence 332 gives the maximum Yvalue for rectangle 311, of 1FF hex. This word also ends the descriptionof frame 301.

Sequence 333 is output by Preprocessor2 20B to Filler Module 2 23B, anddefines the rectangle 312 for frame 302. Sequence 333 is structured insimilar fashion to sequence 332, and can easily be understood from theexplanation of sequence 332.

Figure Filler to Pixel Memory

The 22 bit words 331 sent from Preprocessor1 to Filler Module1 23A andfrom Preprocessor 2 to Filler Module2 23B, are transformed from vectorrepresentations of polygons within frames 106, to bit-mapped,filled-figure representations. The bit-mapped, filled-figurerepresentations of the polygons are output from the Filler Modules asgroups of 25 bit Filler words 339 which are received by the Pixel MemoryModules. As seen in FIG. 9, the 25 bit Filler words 339 output by FillerModule1 23A are received by Pixel Memory Module1 24A, and the Fillerwords 339 output by Filler Module2 23B are received by Pixel MemoryModule2 24B.

A Filler Module generates a bit-map of a frame 106 by indicating the X-Ycoordinates (addresses) of all the pixels 119 on and in the interior ofeach polygon within a frame 106. For each row 120 of 1024 pixels in aframe, the X-Y coordinates of each pixel which lies on or in a polygon,is indicated by one or more of the 25 bit Filler words 339. If a pixel119 does not lie in or on a polygon, then there will be no 25 bit wordto indicate its X-Y coordinates. It is the pixels whose X-Y coordinatesare indicated that subsequently will be illuminated by the laser beam asit raster scans across the rows 120. During inspection, it is thesecoordinates which will be compared with the actual pattern on the target103. If the X-Y coordinates of a pixel are not indicated, then thatpixel does not lie on or within a polygon. A single 25 bit Filler wordcan indicate the X-Y coordinates of all the pixels in a group of 1, 16or 64 adjacent pixels, by giving the X-Y coordinates of the left-mostpixel in the group. All of the pixels in one of these addressed groupswill be in the same row 120. As can be seen in FIG. 35, the X coordinateis given as a hexadecimal number in bits 0-9, the X coordinate field 340of the 25 bit Filler word. Bits 10-19, the Y coordinate field 341,contain a hexadecimal number which is the Y coordinate of the row inwhich the addressed group of pixels is to be found. Bits 20-24, thecommand field 342, are control bits conveying command or controlinformation.

The command field 343 of one of the 25 bit words 339, is used toindicate whether that word is a command word and its meaning, or itindicates the meaning of the X coordinate field 340 and the Y coordinatefield 341 of that word. For example, 16 hex in the command field 342indicates that word is a command word meaning that the end of a frame106 has been reached. The end-of-frame command word is the last 25 bitword in a group of 25 bit words describing the polygons in a singleframe.

01 hex in the command field 342, indicates that the X and Y coordinatefields of that word contain the X and Y coordinates of the leftmostpixel in a group consisting of one pixel. 02 hex in the command field342, indicates that the X and Y coordinate fields of that word containthe X and Y coordinates of the leftmost pixel in a group consisting of16 adjacent pixels in a single row 120. 03 hex in the command field 342,indicates that the X and Y coordinate fields of that word contain the Xand Y coordinates of the leftmost pixel in a group consisting of 64adjacent pixels in a single row. Thus, if fewer than 16 pixels lie on orin a polygon, the bit-map is constructed by addressing each such pixelwith a word whose command field 342 contains 01 hex. Likewise, groups ofadjacent pixels between 16 and 63 in number, are addressed bycombinations of 25 bit words that each address 1 or 16 adjacent pixels.For example, 35 adjacent pixels in a row could be addressed by anycombination of two 25 bit words whose command field is 02 hex, and three25 bit words whose command field is 01 hex. For groups of adjacentpixels greater than 64 in number, 25 bit words whose command fieldcontains 03 hex would be used in combination with words whose commandfield 342 contains 02 hex and words whose command field is 01 hex, asnecessary to add up to the total number of pixels needing to beaddressed. Only adjacent pixels are possible to address with a single 25bit word. If two pixels are in the same row but are separated by one ormore pixels that do not lie in or on a polygon, then those two pixelscannot be addressed by the same 25 bit word.

In each row 120 the Filler Modules 23A and 23B each process the left andright vectors to determine the leftmost pixel which must be addressedand the rightmost pixel which must be addressed, by the 25 bit words339. The leftmost pixel in a given row will correspond to theintersection point of a left vector with the row (only one left vectorper polygon will intersect any given row). Similarly, the rightmostpixel in the same given row will correspond to the intersection point ofa right vector with the row (only one right vector per polygon willintersect any given row). Pixels 119 are located at discrete pointsalong a row, thus, each vector probably will actually intersect anygiven row at a position which is somewhere between, rather thancoinciding with, one of two adjacent pixels. If the actual point ofintersection does not coincide with a pixel position, the Filler Modulesexecute Bresenham's Line Algorithm to determine which of the twoadjacent pixels is nearest to the actual point of intersection. Thenearest pixel's X-Y coordinates will be stored as being the given row'sleftmost (or rightmost, as appropriate) pixel address. Bresenham's LineAlgorithm is well-known in the art, and among other places, can be foundat Page 433-435 of the text "Fundamentals of Interactive Graphics", byJ. D. Foley, published by A. Van Dam, 1982, which by reference isincorporated herein. In the example illustrated in FIGS. 26 and 33, theleft vector 327 and the right vector 328 each have row intersectionpoints which all coincide exactly with pixel positions.

FIG. 35 is a list of the 25 bit data words 339 for bit-mapping therectangle 311 in frame 301. This frame of words is output by FillerModule1 23A to Pixel Memory Module1 24A. As can be seen from FIGS. 26and 32, rows 119 having Y coordinates of 000 through OFF hex, do notcontain any pixels lying on or within the rectangle 311. In other words,none of the pixels in those rows lie on or between left vector 327 andright vector 328. Therefore, there are no 25 bit words with a Ycoordinate of 000 through 0FF hex.

The first row to have pixels 119 on or within rectangle 311, is the rowhaving a Y coordinate of 100 hex. These pixels comprise the pixelscorresponding to the turnpoints 303 and 304 and all pixels in that rowin between the turnpoints. Turnpoint 303 has coordinates X=100 hex andY=100 hex, and is the first pixel in that row whose coordinates areindicated by one of the 25 bit words 339. As seen in FIG. 35, the group343 of five 25 bit words indicates the X-Y coordinates of the first rowof pixels for rectangle 311. That row has a Y coordinate equal to 100hex, and the leftmost group of 64 pixels in that row has the Xcoordinate of the leftmost pixel in that group given as 100 hex in the Xcoordinate field 340 of the first 25 bit word in group 343. The Ycoordinate field 341 contains 100 hex, which is the Y coordinate of eachpixel in the leftmost group.

The second 25 bit word in group 343 gives, in the X coordinate field340, the X coordinate of the leftmost pixel of the second group of 64pixels. This X coordinate is 140 hex and is 40 hex (64 decimal) greaterthan the X coordinate 100 hex of the previous group of 64 bits.Likewise, the next two 25 bit words give the X coordinates of the nexttwo groups of 64 pixel groups.

The first four 25 bit words in group 343 each have 03 hex in the commandfield 342 to indicate that the X-Y coordinates given are of the leftmostpixel in a group of 64 pixels.

The fifth and last 25 bit word of group 343 has a 01 hex in its commandfield 342, indicating that this word will give the coordinate of theleftmost pixel in a group consisting of a single pixel. This 25 bit wordcompletes the addressing of all the pixels of the Y=100 hex row. Allfive 25 bit words in group 343 have 100 hex in their Y coordinatefields. The first four 25 bit words of group 343 each address 40 hexpixels, so the four words together address 100 hex pixels. The fifthword addresses one pixel, giving a total of 101 hex pixels for each rowof rectangle 311. This may also be seen in FIGS. 26 and 33.

The group 344 of five 25 bit words address the 101 hex pixels in thenext row, where Y=101 hex. This group 344 of five words is structured insimilar manner to the first group 343.

FIG. 35 does not show the groups of 25 bit words for Y=102 hex throughY=1FE hex, but they are structured in similar manner to groups 343 and344.

The last group 345 for frame 301 is shown in FIG. 35 and is similar instructure to the groups 343 and 344. All five words in group 345 have1FF hex in their Y coordinate field.

The last 25 bit word in the group 339 of words defining the rectangle311 in frame 301, has a 16 hex in its command field 342. No data iscontained in the Y coordinate field 341 or in the X coordinate field340. This word is a control word indicating the end of the 25 bit wordsfor the frame.

FIG. 36 shows the 25 bit words 339 output by Filler Module2 23B to PixelMemory Module2 24B, describing rectangle 312 in frame 302. The structureof these 25 bit words is similar to those shown in FIG. 36. The onlydifference is that the words shown in FIG. 36 are for rows 120 rangingin Y coordinates from 000 hex to 0FF hex.

Pixel Memory Modules to High Speed Laser Interface Module

FIG. 37 shows the final bit-map 347 of the entire frame 301, includingthe pixel locations which are not to be illuminated by the laser beam asit raster scans the rows of the frame. The bit-map shown in FIG. 37resides in the output memory of Pixel Memory Module1 24A, as a sequenceof 64 bit words at memory addresses 346. The sequence of 64 bit words isoutput from the output memory to the High Speed Laser Interface Module27. Each 64 bit word is output in parallel on a 64 bit bus to Module 27.The ECL High Speed Laser Interface Module 28 receives the 64 bit wordsfrom Module 27, and produces a serial bit stream by using a shiftregister to do parallel to serial conversion of the 64 bit words. Theserial bit stream is input by the Vendor Laser Scan Electronics 348,which is a part of the Electro-optical/Acousto-optical laser opticalsystem 37. The serial bit stream is used by the Vendor Laser ScanElectronics 348, and specifically the electro-optical modulator 101, tomodulate a laser beam to illuminate selectively pixels on target 103.The laser beam raster scans every pixel position on target 103, and ifthe electro-optical modulator 101 receives a bit equal to one when thebeam is over a given pixel position, then that pixel will be illuminatedby the laser beam. If the bit received by the electro-optical modulator101 is a zero rather than a one, then the electro-optical modulator willturn the beam off when it is over the given pixel position.

The bit-map 347 has a bit corresponding one-to-one for each pixel ontarget 103 surface 121. A zero in a bit position of any of the 64 bitwords of the bit-map means that the corresponding pixel on the surface121 will not be illuminated by the laser. A one in a bit position of anyof the 64 bit words of the bit-map means that the corresponding pixel onthe surface 121 will be illuminated by the laser as it raster scans thepixel's position.

As seen in FIG. 37, Pixel Memory addresses 0000 hex through 0FF hex allcontain 64 bit words having a zero in every bit position. There are 1000hex words in this portion of memory, and 40,000 hex bits. Each row 120has 400 hex (1024 decimal) bits, therefore, 40,000/400 gives 100 hexrows represented by Pixel Memory addresses 0000-0FFF hex. Thus, the wordat Pixel Memory address 1000 hex corresponds to the leftmost 64 pixelsof row Y=100 hex, which is the first row in which pixels will beilluminated to construct rectangle 311 in frame 301. Since the turnpoint303, FIG. 26, has X coordinate equal to 100 hex, the first 100 hexpixels in that row will not be illuminated by the laser. Thus, the four64 bit words in Pixel Memory positions 1000 hex through 1003 hex willcontain all zeros. The next pixel will correspond to turnpoint 303 atX=100 hex. There are 101 hex pixels from X=100 hex through turnpoint 304at X=200 hex. Therefore, four 64 bit words plus one bit are required inwhich each bit position contains a one. The four 64 bit words with eachbit equal to one are the words in Pixel Memory addresses 1004-1007 hex.The extra bit equal to one is found in bit 63 of the 64 bit word atPixel Memory address 1008 hex. Since the strip 105 has X addresses from000 through 3FF hex, There are 200 hex pixels in the range X=200 hexthrough 3FF hex. Thus eight 64 bit words are required to describe thisportion of the row Y=100 hex. The word at 1008 hex has bit 63=1, asdiscussed above, with bits 0-62 being zeros. The 64 bit words ataddresses 1009-100F hex have zeros in all their bit positions,completing the description of row X=100 hex.

The description of row Y=101 hex begins with the 64 bit word at PixelMemory location 1010 hex, and continues in the same manner as for rowX=100 hex.

The description of rows Y=102-1FF hex will be the same as for row Y=100hex.

The bit map for frame 302 is not shown, however, it will be structuredin a similar manner to frame 301, as shown in FIG. 37. The rectanglebegins with row Y=000 hex and ends with row Y=0FF hex. Rows Y=100-1FFhex do not contain any pixels in or on a polygon, therefore, all the 64bit words describing this area of the frame 302 have zeros in all theirbit positions.

PATTERN INSPECTION Pattern Illumination and Detection

As seen in FIGS. 41-43, pattern inspection can be done on any target 103having a pattern 422 on a background 426, wherein either, but not both,the pattern 422 or the background 426 specularly reflects incident laserlight, and the other diffusely reflects incident laser light. Thus,optically, the pattern 422 and the background 426 differ from each otherby the manner in which they reflect an incident laser beam. This meansthat the pattern 422 can be distinguished from the background 426 bydetecting the presence or absence of laser light which has beenreflected at an angle larger than would be the case with specular ornearly specular reflection. Inspection can be performed most accuratelyon those targets whose patterns and backgrounds differ the most withrespect to specular versus diffuse reflectivity.

In the example illustrated in FIG. 43, the pattern 422 specularlyreflects incident laser light and the background 426 diffusely reflectslaser light. Incident laser beam 414 is normal to the surface of pattern422 and reflects back upon itself as shown at 423. Thus, the surface ofthe pattern 422 is shown producing perfectly specular reflection.Incident laser beam 424 is normal to the surface of background 426 andis scattered at many angles as indicated by reflections 425. Thus, thesurface of the background 426 produces diffuse reflection.

An example of a target 103 which would produce the reflections shown inFIG. 43 is a printed wiring or printed circuit board having coppercircuit patterns produced by etching away unwanted copper from the faceof an epoxy board which had been entirely covered with a thin layer ofcopper. Printed wiring boards have patterns 422 which are shiny,specularly reflective copper circuit patterns, while the background 426is epoxy board having a diffusely reflective surface.

Another example of a target such as the one shown in FIG. 43 is an epoxyboard having a thin layer of copper on its face, and photoresistpatterns 422 on the surface of the copper. The photoresist had coveredthe copper surface entirely and was then etched away selectively toproduce the photoresist pattern 422. The etching process also etches thesurface of the background copper 426 sufficiently to cause the surfaceto reflect light diffusely. The photoresist pattern 422 has a relativelyshiny surface which causes incident laser light to be reflected in asubstantially specular manner.

FIGS. 41 and 42 show an arrangement of four fiber-optical detector heads416 surrounding the objective lens 117, to detect scattered or diffuselyreflected laser light. Objective lens 117 is shown in the context of theoptical system in FIG. 5. The optical system shown in FIG. 5 is used inthe pattern inspection system 400 as well as in the pattern writingsystem 50. FIG. 5 does not show the fiber-optical detector system 413.

The fiber-optical detector system 413 includes the four fiber-opticaldetector heads 416, the fiber-optical cables 420 which join together ascable 421, and photomultiplier tube 45. Each fiber-optical cable 420 isa circular arrangement of a plurality of individual fiber-optic fibers.Head 416 terminates cable 420 in a thin, wide, rectangular array of theindividual fiber-optic fibers. The width 427 of a fiber-optic head 416is indicated in FIG. 42. The optical aperture 428 of head 416 is the endof the thin, wide, rectangular array of the individual fiber-opticfibers, through which light is admitted to travel down a cable 420 intocable 421 and into the photomultiplier tube 45, where an analogelectrical signal is generated whose amplitude indicates the intensityof the light. Fiber-optical cable 413 is a circular arrangement of theindividual fiber-optic fibers from each of the cables 420. The lightentering photomultiplier tube 45 is the sum of the light entering theoptical apertures 428 of the four heads 416. In the example shown inFIG. 43, an electrical signal is generated by the photomultiplier tube45 when laser beam 424 strikes background 426 and is diffusely reflectedand intercepted by the apertures 428 of heads 416. When laser beam 414strikes pattern 422, the reflected light beam 423 returns along the samepath and is not intercepted by apertures 428 of heads 416, and noelectrical signal is generated. The electrical signals may be invertedif desired so that the presence of a signal indicates the presence ofpattern 422, and the absence of a signal indicates the absence of anypattern.

As seen in FIGS. 41 and 42, the four fiber-optical heads are radiallydisposed about the optical axis 417 of objective lens 117, and arelocated at the four vertices of a square centered upon and normal to theoptical axis 417. Arrow 429 in FIG. 42 indicates the direction that arow 120 will be raster scanned by the laser beam passing throughobjective lens 117. A row is very short so that the angle subtended bythe row will be close to zero. Thus, although the scanning laser beamwill not always be quite normal to the surface of target 103, specularreflection will produce a reflected beam in which the angle of incidenceplus the angle of reflection is sufficiently small to prevent thereflected beam from entering the optical aperture 428 of any of theheads 416. Each fiber-optical head 416 is aligned so that its opticalaperture 428 points at the region where a laser beam will strike thesurface of target 103. This can be accomplished by pointing theapertures at the point where optical axis 417 intersects the surface oftarget 103, as shown in FIG. 41. It is, however, not required that allthe apertures point to a single point, but each may point to a distinctpoint somewhere along or near the row to be raster scanned.

Instead of being disposed at the vertices of a square, the fourfiber-optical heads 416 may be disposed at the vertices of a rectangle.

Diagonally opposed pairs of fiber-optical heads 416 are disposed at theproper height above the surface of target 103 so that the angle 415subtended by their distance apart is a right angle. This may be seen inFIG. 41, where reflected light beams 418 and 419 are reflected from apixel 119 at right angles to one another, and enter the opticalapertures of the two fiber-optical heads 416 shown. This angle allowsthe apertures 428 of the heads to be sufficiently far from optical axis417 so that specularly reflected light will not enter the apertures 428.Also, an angle 415 of ninety degrees is small enough to optimize theinterception of diffusely reflected light, if there is any. An angle ofninety degrees works well for light diffusely reflected from epoxy boardor etched copper. This angle can be varied if required for optimuminterception of diffusely reflected light from other types of surfaces.

The Electronics

FIG. 38 is a block diagram of a pattern inspector system 400, which alsois capable of writing, as described previously. The pattern inspectorsystem shares many of the functions of the pattern writer system 50,shown in FIG. 6. The pattern inspection system 400 is another embodimentof the invention and will be described in terms of the additions anddifferences with the writer.

For the most part, the blocks numbered 1-11 function as described inconjunction with FIG. 6. The host computer 3 functions to generate acommand list to inspect when the system 400 is functioning as aninspector, as well as to generate a command list for writing when thesystem is functioning as a writer, as described in conjunction with FIG.6. During inspection, the host computer 3 also does sorting of areceived error list which identifies the X-Y coordinates of those areasof a detected pattern on the target 103 surface which are part of thedetected pattern but should not be present, or those areas of the target103 which should have patterning but in fact do not. The host computer 3stores the X-Y coordinates of the sorted list in host memory 4.

The database resident on tape 17 for the inspection system 400 is thesame as described for the writer system 50, or for the inspection system400 when in the writer mode. At this point, the database has onlyreference polygons in each frame of the database. The reference polygonsform the ideal patterns against which the actual patterns on the target103 will be compared. The reference polygons also are the ideal patternswhich are written operation.

As with the writer system 50, DPC2 15 reads the database from tape 17through tape drive controller 16, reformats it into turnpoint polygonrepresentation, as before, and stores the turnpoint polygonrepresentation of the database on data disk 13 through disk controller12. As part of the inspection system 400, DPC2 15 also generatesguardband turnpoint polygons for the reference polygons in each frame106. The guardband polygons form a don't care zone around each referencepolygon side. During inspection of an actual part, mismatches betweenthe actual pattern on a target 103 and the ideal pattern described bythe database are not flagged as errors if the mismatches occur in adon't care zone, i.e., within a guardband polygon. The guardband aroundeach reference polygon side will be narrow enough so that unsatisfactorypatterns will not be passed as being good, and wide enough so thatinsignificant variances between the ideal pattern and the actualpattern, will not cause the pattern on the target to be rejectedunnecessarily.

Each side of a polygon is a line segment completely contained within asingle frame 106. Preferably, a guardband polygon is a rectanglesurrounding the line segment, such that the perpendicular distance fromany point on the line segment to the nearest point on the guardbandrectangle, is a fixed, small distance epsilon. Typically, epsilon willbe a distance equal to a small number of pixels. Thus, the guardbandrectangle will have two sides parallel to and equidistant from the linesegment. Preferably, the other two sides of the guardband rectangle willeach be a distance epsilon, measured from their midpoints, from thenearest endpoint of the line segment.

A guardband polygon will not always be a rectangle, nor will it alwaysbe exactly as previously described. When the line segment is near or onone of the frame boundaries, the distance epsilon may be greater thanthe distance of all or some portions of the line segment from the frameboundary. As is the case with reference polygons, a guardband polygonmust lie completely within, or on, a single frame 106. Thus, anyportions of the guardband rectangle which otherwise would protrude intoan adjacent frame, instead are truncated by the frame boundary, and thetruncating portion of the frame boundary forms one of the sides of theguardband polygon.

FIG. 47 shows the guardbands surrounding the sides of the rectangle 300,the example pattern illustrated and discussed in connection with FIG.26. The rectangle 300 requires six guardband polygons, i.e., sixguardband rectangles. Since the rectangle 300 straddles the upper frame301 and the lower frame 302, and since guardband polygons cannot crossframe boundaries, the left and right sides of the rectangle 300 willeach have two guardband polygons (rectangles) instead of only one. Thatis, the left side of rectangle 300 will be surrounded by guardbandrectangle 248 in frame 301, and by guardband rectangle 251 in frame 302.Similarly, the right side of rectangle 300 will be surrounded byguardband rectangle 249 in frame 301, and by guardband rectangle 253 inframe 302.

The top side of rectangle 300 is completely contained within frame 301,and so, will have only one surrounding guardband rectangle 250.Likewise, the bottom side of rectangle 300 will have only onesurrounding guardband rectangle 252.

In frame 301, guardband rectangle 248 etends from X=0FD hex to X=103hex, and from Y=0FD hex to Y=1FF hex. Guardband rectangle Y=1FF hex.Guardband rectangle 250 extends from X=0FD hex to X=203 hex, and fromY=0FD hex to Y=103 hex.

In frame 302, guardband rectangle 251 extends from X=0FD hex to X=103hex, and from Y=00 hex to Y=102 hex. Guardband rectangle 252 extendsfrom X=0FD hex to X=203 hex, and from Y=0FC hex to Y=102 hex. Guardbandrectangle 253 extends from X=1FD hex to X=203 hex, and from Y=00 hex toY=102 hex.

As can be seen, guardband rectangles 248 and 250 overlap in the upperleft corner, and guardband rectangles 249 and 250 overlap in the upperright corner. Similarly, guardband rectangles 251 and 252 overlap in thelower left corner, and guardband rectangles 252 and 253 overlap in thelower right corner.

It is seen that each of the surrounded guardband rectangles has sidesspaced a distance epsilon=3 pixels from the surrounded side of rectangle300.

From FIG. 47, it can be seen that guardband rectangles 248 and 251, abutframe boundary 318. Together, guardband rectangles 248 and 251completely surround the left side of rectangle 300. Guardband rectangle248 end at Y=1FF hex in frame 301, and guardband rectangle 251 begins atY=00 hex in frame 302. Between Y=1FF in frame 301 and Y=00 hex in frame302, there is no gap or omitted pixel position, since Y=1FF hex is thelast Y address in frame 301. A similar situation exists with respect toguardband rectangles 249 and 253.

As with the writer system 50, DPC3 14 reads the database from data disk13, inserts Move To and Draw commands into the database, and outputs themodified database into the pipelines. Unlike the writer system 50,however, DPC3 14 sends all the guardband words, frame-at-a-time, downthe bus out1 401 to pipeline buffer1 18A, and all the reference words,frame-at-a-time, down the bus out2 402 to pipeline buffer2 18B. As withthe writer system 50, alternate frames of words are sent topreprocessor3 20C and preprocessor4 20D. Similarly, frames of guardbandwords are alternately sent to preprocessor1 20A and preprocessor2 20B.Window clipper 19 functions as with the writer system 50. The embodimentof the inspection system 400 shown, has four pipelines which begin withthe four preprocessors 20A-20D.

Preprocessors 20A-20D, filler modules 23A-23D, pixel memory modules24A-24D, and stage controller 25 function as described for writer system50.

The pixel memory interface module 53 functions like the high speed laserinterface module 27, except that a memory buffer for the guardband wordsis also included. The reference and the guardband words are output tothe ECL comparator 40.

The ECL comparator 40 has a shift register which converts the 64 bitreference data words from the pixel memory interface module 53 intoserial reference data. Likewise, the ECL comparator 40 has a shiftregister which converts the 64 bit guardband data words from the pixelmemory interface module 53 into serial guardband data.

Photo tube 45 receives laser light reflected from the pixels on thesurface of target 103 and the pattern thereon, and onverts the reflectedlight into an analog scan signal which is proportional to the intensityof the light received by the tube 45. An unmodulated laser beam rasterscans the surface of target in the same manner as for writing. The onlydifference is that the laser beam is unmodulated and of a constant,predetermined intensity during inspection. The analog scan signal isamplified by amplifier 44, and the output of the amplifier is receivedby converted by an analog-to-digital-converter to a digitalrepresentation of the reflected light intensity at a pixel on thesurface of the target 103. The digital scan signal is clocked into ascan latch at the precise intervals during raster scanning so that thelatched digital scan words are the digital representations of thereflected light intensity at each scanned pixel location. The digitalscan word is a gray scale representation of the reflected lightintensity. The digital scan word is sent to a digital comparator whichcompares the digital value with a predetermined, stored threshold value.If the digital scan word is greater than or equal to the thresholdvalue, the comparator outputs a single bit which has the value of one.If the digital scan word is less than the predetermined threshold value,then the comparator outputs a single bit which has the value of zero. Asthe laser beam raster scans the target 103, 1024 digital scan words areclocked into the scan latch for each row scanned. Each digital scan wordrepresents the reflected light intensity at one of the pixels in thescanned row. Thus, the comparator generates scan data comprising aserial bit stream consisting of 1024 bits for each raster scanned row,where each bit is a binary representation of the reflected lightintensity at one of the 1024 pixel positions in the scanned row.

An alternate embodiment for the pattern inspector system 400 is used forinspection of patterned, transparent targets, such as film or glassphotomasks or reticles. With this embodiment, the mask is inspected bytransmission of laser light rather than by reflection. The back of themask (target 103) is illuminated with a collimated uniform light sourcethat illuminates the imaged area of the target. Imaging can be doneeffectively by the use of a CCD line scanner, particularly, a segmentedscanner with multiple output such as the EG&G Reticon 1024 elementscanner which is subdivided into eight contiguous line scanners eachcomprised of 128 elements. These 128 element sections are organized aseven and odd serial pixel outputs. That is, during the first clockpulse, signals for the first and second pixels are output in parallel;during the second clock cycle, signals for the third and fourth pixelsare output in parallel; and so on.

The scanner is driven by parallel clocks for each section, so thatduring the first clock cycle, pixels 1 and 2, 129 and 130, 257 and 258,385 and 86, 513 and 514, 641 and 642, 769 and 770, and 897 and 898 areoutput at the same time over 16 output lines. This increases theeffective data rate, or bandwidth, to more than 240 megapixels persecond, given sufficient illumination intensity. Each of these analoggrayscale pixel outputs is sampled to convert the CCD current pulseoutput into a constant level which is proportional to the lightintensity falling on the corresponding pixel.

The sixteen sampled and held parallel pixel outputs individually arecompared with the threshold reference level. These latched parallelanalog outputs have been trimmed individually to reduce or eliminatedifferences in the efficiency of the individual CCD output channels.After threshold comparison, the resulting bit-per-pixel binaryinformation is stored until a row 120 of 1024 pixels has been built. Theparallel 1024 bit data is double buffered to permit overlayed dataacquisition and readout. This is useful for continuous stage 35 motionand synchronization. The 1024 bits of data are read out serially inleft-to-right order relative to the scanned row. This output isanalogous to the output from the photo tube 45 of FIG. 38. Readout inthe light transmission embodiment is synchronized with the stage 35motion and is clocked synchronously with the integrating detectors overthe row scan time rather than a pixel time, the system requires betterstage 35 velocity uniformity than the fixed pixel illumination time ofthe acousto-optically scanned laser system 400 used for reflectedinspection.

For situations that require serial data rates for which adequatelybright light sources are not available for imaging with an integrationtime of one scan row 120, a time integrating imaging system can be used.In one embodiment of this technique, a conventional area CCD imager isused rather than a row scanner detector. With this approach, the storedcharge is moved from row to row of the scanner, broadside (in parallel)at the same speed as the inspected image moves over the sensitive areaof the scanner.

The effect of the time integrated detection is to increase the effectiveintegration time from one-row scan-time, to the scan time for all therows in the scanner. Typically, a one thousand fold increase ineffective sensitivity can be achieved with this technique. A CCD areasensor such as the TI MC780 can be used in this manner and can takeadvantage of the triple output, normally provided for use with colorcamera applications. The area sensors either directly (for single outputimagers) or after interleaving for multiple output imagers, are outputin buffered serial form for comparison in the same manner as describedfor reflective or row scanner light transmission systems.

These same imaging techniques can be extended to other forms of databasereferenced inspection, such as two or three dimensional x-rayinspection. The area sensor approach combined with either a microchannelor conventional x-ray image intensifier is effective particularly as adetection system.

The serial bit stream of scan data is compared bit-by-bit with theserial reference data and the serial guardband data to detect errors, ordefective areas of the actual pattern on target 103. Each bit positionbeing compared corresponds to a single pixel on the target surface andcorresponds to the same pixel in the reference data and in the guardbanddata. The compare is a logical operation done by Exclusively ORing(XORing) a bit from the serial scan data with the corresponding bit fromthe serial reference data, and then ANDing the result with the onescomplement of the corresponding bit from the serial guardband data, toget the error bit. If the result (the error bit) of the logicaloperation is a one, then the scan bit's corresponding pixel's X-Yposition is flagged as being an error location on the target 103. Theerror is flagged as being one of having a portion of pattern present atthat pixel on target 103 when it should not be there, if the scan bit isa one. The error is flagged as being one of not having a portion ofpattern present at that pixel on target 103 when it should be there, ifthe scan bit is a zero. If the error bit is a zero, then there is noerror at the corresponding pixel position on the target 103 and there isno error in that bit's corresponding pixel on target 103. Thus, thepattern is correct at that pixel.

As error data is first detected, the corresponding starting X-Ycoordinates and scan data bit are latched. The starting X-Y coordinatesare the X-Y coordinates of the first pixel, on the target 103, of asequence of pixels generating error bits equal to one. When the length,in number of pixels, i.e., number of incoming scan bits, of the errorsequence has been determined to be at least as long as a predeterminedminimum recordable error length, the starting X-Y coordinates, errorlength and starting buffer. The minimum error length which will be savedis a predetermined value and represents the minimum length, in number ofpixels, i.e., number of incoming scan bits, of a detected inspectionerror in the pattern on the surface of target 103, in order for thaterror to be stored for operator analysis. Errors shorter in length thanthe predetermined minimum are ignored by the inspection system 400, andare not saved in the FIFO memory error buffer in the ECL Comparator 40.An error length counter begins once an error sequence has been detected.The counter continues to count incoming scan bits until the number ofconsecutive good scan bits with error bit equal to zero, also equals thenumber representing the minimum recordable error length. A good scan bitis one whose corresponding error bit is equal to zero.

The X-Y coordinates of the first bit in an error sequence long enough tobe saved in the FIFO, are saved in the FIFO memory error buffer alongwith the polarity of the scan bit. The FIFO memory error buffer allowsfor a smooth transfer of error information to the Error Buffer module41, seen in FIG. 38. Upon availability of the error information, theError Buffer module performs asynchronous reads from the FIFO.

The FIFO memory error buffer is 41 bits wide and 256 locations deep.Each 41 bit location holds 10 bits of X coordinate, 20 bits of Ycoordinate, 10 bits of error sequence length, and a scan bit. The scanbit denotes that a section of pattern is missing if the scan bit iszero, and denotes that the pattern has additional area if the scan bitis one.

The Error Buffer module 41 is a 4K by 48 bit FIFO which serves as anintermediate storage between the ECL Comparator 40 and bit-sliceprocessors in the Error Consolidator module 42. The Error Buffer module41 reads the error information from the ECL Comparator 40 and places itin the 4K by 48 bit FIFO so that the Error Consolidator 42 processorscan retrieve the information as required. The Error Buffer module 41operates asynchronously with respect to both the ECL Comparator 40 andthe Error Consolidator module 42.

Error Consolidator 42

The Error Consolidator 42 includes a 24 bit bit-slice processor whosefunction includes the task of grouping associated sequences of errorsdetected in the pattern on target 103 by the ECL Comparator 40. Theerror data is organized into one or more consolidated error defectareas. The error data is the collection of saved data describing, orflagging, those pixels whose X-Y coordinates have been saved by the ECLComparator as being locations of missing or additional areas of thepattern on target 103.

The Error Consolidator also eliminates classes of false errors generatedby the laser beam and optical system, and by inaccuracies in convertingthe digital representation of the reflected light intensity into abinary, single-bit representation. Also, closely spaced sequences ofdefective pixels are united into single defect areas by comparing theseparation of defective sequences of pixels with an error criterionallowance.

The error criterion allowance is the minimum number of adjacent pixelsin the X or Y direction that must be miscompared between reference dataand scan data, and not lie within a guardband, before an error isreported to DPCI 56.

As discussed above, the error criterion allowance in the X direction isapplied by the ECL Comparator 40 to test a sequence of adjacentdefective pixels for the requisite minimum length.

The error criterion allowance in the Y direction can be applied, as inFIG. 39, by the Error Consolidator 42 to ensure that a predeterminedminimum, the error criterion allowance, of contiguous rows of sequencesof error-flagged pixels overlap in the X direction, before the sequencesof error data are consolidated and reported to DPCl 56 as a singledefect area.

FIG. 39 shows a section of a frame 106 on the surface of target 103.Also shown is an example of a defect area 404 comprising pixels 119whose X-Y coordinates have been saved as sequences by the ECL Comparator40 due to miscompares between the scan data from these pixels and thereference data. Also, these pixels do not lie within any of theguardbands around the sides of the reference data polygons.Notwithstanding the fact that the ECL Comparator 40 has saved the datadescribing these pixels as being in a defect area, the ErrorConsolidator 42 will not report these pixel locations as containingdefects unless the number of adjacent rows of sequences of pixelsoverlapping in the X direction is greater than the predetermined errorcriterion allowance. Defect areas, such as at 404, having a delta Y 403(number of overlapping rows of sequences) which is less than or equal tothe error criterion allowance, are considered false errors and are notreported as defect areas to DPC1 56. In the example of FIG. 39, delta Y403 equals 4. In order for the area 404 to be reported as a consolidateddefect area to DPC1 56, the predetermined error criterion allowance mustbe the number three or less. Otherwise, the area 404 will be considereda false defect area, and will not be consolidated or reported.

Error data which is consolidated as a defect area and reported to DPC156, is described by six parameters, which are: a 10 bit X minimum, a 10bit X maximum, a 20 bit Y minimum, a 20 bit Y maximum, a 23 bit areaexpressed as the number of defective pixels in the defect area, and asingle scan data bit indicating whether the defect area represents addedor missing parts of the pattern on target 103. X minimum is the Xcoordinate of the leftmost pixel in the defect area. X maximum is the Xcoordinate of the rightmost pixel in the defect area. Y minimum is the Ycoordinate of the topmost pixel in the defect area. Y maximum is the Ycoordinate of the bottom-most pixel in the defect area. These sixparameters are sent to DPCl 56 for each consolidated, reportable defectarea.

For the example shown if FIG. 39, X minimum=5, X maximum=13, Yminimum=1, Y maximum=4, and the area=22 pixels. The scan data bit is notindicated as to whether it is a one or a zero.

The error criterion allowance can also be applied to consolidate into asingle defect area, non-contiguous sequences of defective pixels closelyspaced in the X or Y direction.

For example, in FIG. 40, the leftmost defect area 405 and the rightmostdefect area 406 consolidate into a single defect area provided that atleast one of the sequence separation distances, 408, 409 or 410 is lessthan or equal to the error criterion allowance. Furthermore, defect area405 and defect area 406 must each have a sufficient number ofoverlapping adjacent rows of error sequences to be consolidatable as adefect area. That is, the number of overlapping rows of error sequencesmust be greater than the error criterion allowance.

Similarly, the bottom-most defect area 407 consolidates with defectareas 405 and 406 if areas 405-407 are separately consolidatable asdefect areas, and if the defect separation 411, measured by the numberof intervening rows, is less than or equal to the error criterionallowance. The bottom-most defect area 407 must also have its errorsequence in row Y=7 overlap in the X direction with the two errorsequences in row Y=5.

DPC1 56

DPCI 56 includes a bit-slice processor to do fast sorting of the datadescribing the defect areas consolidated by the Error Consolidator 42.The defect areas are sorted by the area in pixels, in descending orderand formed into a list which is sent to host memory 4 for use by thehost computer 3.

Modules 30-36 are the same as described in conjunction with FIG. 6.

Operator Review of Defect Areas

The list of sorted errors sent to the host computer 3 by DPC1 56 ispresented on the video display terminal 2 for review by the humanoperator. The list of errors, that is, defect areas, is presented inorder of the most serious, i.e., those with the largest area first. Theoperator selects which defect area he desires to view on color display29. The stage 35 then moves the selected target defect area into thecorrect X-Y position to be laser scanned in real-time during theoperator review process, to generate a gray scale image of the defectarea. Positioning of the stage 35, and thus the target 103, is computedfrom location information that is a portion of the defect data collectedby the error consolidator 42. The stage is moved to a backoff locationso that it is moving at a constant speed when the defect area to bereviewed is under the raster scanning laser beam.

At this point, the target 103 area, which includes the defect, to bedisplayed is raster scanned by the laser beam and a gray scale image ofthe scanned area is displayed with the defect area centered upon thescreen of the color display 29. The ECL comparator 40 has a six bitanalog-to-digital converter which transforms the analog representationof the reflected light intensity for a single pixel into a six bitdigital gray scale representation. Thus, rather than thresholding eachpixel to a one or a zero as is done during the inspection process, a sixbit range of intensity is stored and then displayed. One of theembodiments of the invention does, however, contemplate that the grayscale image be selectively thresholdable, either between one and zero asduring inspection, or by dividing the 64 possible values for the six bitcombination into a lesser number of values. Each pixel to be displayedmaps onto one and only one display 29 pixel. Each display 29 pixelindividually varies in brightness according to the six bit gray scale,or thresholded, output from the ECL comparator 40.

During the inspection process, when defect areas, if any, are located,only the area in number of pixels, the X-Y extremes and the type ofdefect are saved for each consolidated defect area. Scan data bit-mapsof the defect areas are not saved. This means that the large amounts ofmemory which would be necessary to store bit-mapped images of all thedefect areas discovered during an inspection process, are not required.Also, real-time generation on the fly of a scan data image of a defectarea is quicker and easier than would be the case if a large, bit-map,scan data base had to be accessed and managed.

Overlaying the gray scale image of the scanned area, is a computergenerated reference image of the same area. The computer generatedreference image is generated from the reference data stored on data disk13 on a real-time basis during operator review. The frames to bedisplayed are read from data disk 13 by DPC3 14. The location of desiredframe is found by using a frame look-up table created by DPC2 when thedatabase was read from data disk 13 during the inspection process. Thistable is a directory for finding the beginning of the frame to be read.The data contained in the frame is read by DPC3 and sent down thepipeline that goes to the window clipper 19. This is repeated for eachframe needed to create the overlayed reference image. Usually fourframes are required. If the de-zoom (discussed below) process is notused, from one to four frames of data are possible to view at a time oncolor display 29. Where de-zoom scale of two or greater is used, thenmore than four frames can be viewed on color display 29.

The reference image is created frame-by-frame, on the fly, duringoperator review of the defect area. The data for the displayed frames isread from data disk 13 and changed by DPC3 to the previously discussedmove/draw turnpoint polygon representation of the portion of the patternin the displayed frames. The data is then sent to window clipper 19where only the portion of the data to be displayed by color display 29is sent on to preprocessor1 20A. All frames to be displayed are sentdown the same pipeline to the window clipper 19. Thus, the reviewprocess represents a departure from the writing or inspection processeswhere consecutive frames of data are sent down alternate or differentpipelines. Also, during the review process only a limited number offrames of data will be read from data disk 13 and subsequently sent downthe pipeline. During the review process, guardbands are not generated orused, so the database processing is done in a similar manner to theprocessing of the data during the pattern writing process, with thepreviously noted exception of sending all the frames to be displayeddown the same pipeline.

Preprocessor1 20A creates a vector representation of the frame referenceimage data sent by window clipper 19. Preprocessor1 20A then sends thevector representation of the image data to pixel memory module1 24A,where it is transformed into a representation addressing the pixels onand between left and right vectors. Pixel memory module1 24A thentransforms the image data into a 64 bit word bit-map of the referencedata to be displayed on color display 29.

These reference data transformations are performed on the fly, that is,only the reference data for the defect area being reviewed is read fromdata disk 13 and sent down the window clipper pipeline and subsequentlyis processed and sent to the display 29 on a real-time basis during theoperator review of the defect area. Only the area in pixels, the X-Yextremes and the type of defect are saved for each consolidated defectarea in the scan data. A reference data bit-map of the defect area isnot saved. This means that the large amounts of memory which would benecessary to store bit-mapped reference data images for all the defectareas discovered during an inspection process, are not required. Also,the compacted reference data base on data disk 13 is easier and quickerto access than would be the much larger expanded reference data bit-mapof all the defect areas. Requiring even more memory would be a referencedata bit-map of the entire target surface as produced from the referencedatabase at some point prior to the operator review process.

The overlay produces a yellow color in those areas where the gray scaleimage of the actual pattern and the computer generated reference imageoverlap. Thus, the yellow area of the image on the color display 29represents that portion of the actual pattern on target 103 whichcontains no defects.

The overlay produces a green color in those areas on the target 103where there is pattern present that should not be there.

The overlay produces a red color in those areas on the target 103 wherepattern is absent that should be there.

The color display 29 displays a portion of the target 103 that is 512pixels 119 by 512 pixels 119. A frame 106 is 1024 pixels 119 by 512pixels 119 (FIG. 3), so, at most, only half a frame at a time can bedisplayed on color display 29. Real patterns on a target's surface cancross frame 106 boundaries. However, a reference data representation ofthe same pattern uses contiguous, constituent polygons to represent thepattern. The constituent reference data polygons do not cross frameboundaries but instead, abut against one another at the frame boundariesif the pattern crosses the boundaries. The color display may includeareas from as many as four frames. For instance, the contiguous cornerareas of four frames will be displayed if a strip boundary runsvertically down the middle of the display, and if the horizontalboundaries between the two sets of contiguous frames on either side ofthe strip boundary, run horizontally through the middle of the display29. The window clipper 19 deletes the reference polygons from theportions of the frames which will not be shown on display 29.

The gray scale image of the pattern on target 103 and the reference dataimage of the ideal pattern, can both be de-zoomed. That is, a largerarea of the target and of the reference data can be shown on colordisplay 29, with a corresponding shrinkage in the display size of thetarget and reference data pattern features. De-zoom scales are 1, 2, 4and 8.

With de-zoom scale 1, display 29 shows each pixel 119 that is within theviewing area on target 103, and shows each pixel of the reference datathat corresponds to the viewing area. That is, there is a one-to-onecorrespondence between the pixels of the target viewing area and thepixels of the display 29. Likewise, there is a one-to-one correspondencebetween the pixels of the reference data viewing area and the pixels ofthe display 29.

With de-zoom scale 2, display 29 shows every other pixel of the targetin the X direction, so that the viewing area is now twice as wide as forde-zoom scale 1. Likewise, display 29 shows every other pixel of thetarget in the Y direction, so that the viewing area is now twice as highas for de-zoom scale 1. Therefore, the new viewing area is four times aslarge as for de-zoom scale 1. Similarly, the viewing area for de-zoomscales 4 and 8 are 16 times and 64 times, respectively, as large as forde-zoom scale 1.

Gray scale module 38 receives a 6 bit gray scale word for each pixel inthe target viewing area from the analog-to-digital converter in the ECLcomparator 40. When de-zoom scale 1 is in effect, each pixel wordreceived by the gray scale module 38 is sent to the color display 29.When de-zoom scale 2 is in effect, every other pixel word received fromECL comparator 40 for the first row of the viewing area is sent to colordisplay 29. None of the pixel words received for the second row of theviewing area are sent to the display 29. For the third row of theviewing area, again every other pixel word received is sent to display29, and so on. i.e., every other pixel word from every other viewingarea row of pixel words received from ECL comparator 40 are sent tocolor display 29. For de-zoom scale 4, every fourth pixel word fromevery fourth viewing area row of pixel words received from ECLcomparator 40 are sent to color display 29. Likewise, for de-zoom scale8, every eighth pixel word from every eighth viewing area row of pixelwords received from ECL comparator 40 are sent to color display 29.

Window clipper 19 performs the same de-zoom scaling functions on thereference data in the same way as gray scale module 38 performs de-zoomscaling on the scan data input from ECL comparator module 40.

Autoalignment of the Target

Autoalignment provides three functions for an inspection system 400, orfor a writer system 50 that also has inspection capability:(1)determines the exact location of the target 103;2) adjusts therotational alignment of a stage 35 and target 103 about the theta axisso that the Y direction stage motion follows a strip 105; and (3)determines if the target 103 has thermally expanded or contracted.Autoalignment is achieved by providing fiducial marks in predeterminedlocations on the surface of target 103, and using the inspect capabilityto locate these fiducials after the target has been placed on the stage35.

FIG. 45 shows a target 103 which is a reticle 520 having a top fiducialmark 518 and a bottom fiducial mark 519, each in the center of thereticle with respect to X. The top fiducial mark 518 is shown as a crossand the bottom fiducial mark 519 as a bar, although, any predeterminedidentifiable shapes will work. Top 518 and bottom 519 fiducial marks areshown larger with respect to reticle 520 than would actually be thecase. Normally, each fiducial mark is small enough to fit within asingle frame 1024 pixels wide by 512 pixels high. Preferably, top 518and bottom 519 fiducial marks have centroids with the same X coordinatesin the same center strip 105, so that the reticle 520 will be alignedwhen a line between the centroids of the top and bottom fiducial marksis parallel to the Y axis. It will be understood, however, that theinvention may be practiced with a target having two or more fiducials,at predetermined orientations, that do not have centroids with the sameX coordinates or lie in the same strip, or lie in the center strip.Preferably, each fiducial mark is sized so that it can fit completelywithin a single frame 106.

The alignment process begins by placing manually a target 103 on a stage35. Preferably the stage 35 has an X stop 523 and a Y stop 524 againstwhich the target may be placed to achieve preliminary X-Y-Thetaalignment of the target, where the Theta axis is perpendicular to theX-Y plane. Preliminary alignment means that the X-Y position of thecentroid of each of the top 518 and bottom 519 fiducial marks is knownwithin three adjacent strips 105 and five adjacent frames 106.

The top fiducial mark 518 is located by the pattern inspector 400 in theinspection mode, by raster scanning the middle three strips of thetarget 103, for a Y distance of five frames 106 for each strip 105.During the normal inspection mode, a target 103 is raster scanned instrips which extend for the full, or nearly the full, length of thetarget. In the autoalignment mode, preferably, each strip is only fiveframes long. Also, during the normal inspection mode, a target is rasterscanned in strips which cover, or nearly cover, the entire width oftarget 103. In the autoalignment mode, preferably, only the three centerstrips 105 are raster scanned. Thus, preferably, only the upper fiveframes of each of the center three strips are raster scanned in thereticle of FIG. 45.

Similarly, the bottom fiducial mark 519 is located by the patterninspector 400 in the inspection mode, by raster scanning the bottom fiveframes 106 of each of the center three strips 105 of reticle 520.

Thus, for both top 518 and bottom 519 fiducial marks, only fifteenframes 106 are scanned for each mark. This saves a great amount of timeover scanning the entire target 103, although, that certainly could bedone.

The data flow for inspection to find the fiducial marks proceeds aspreviously described for inspection of the entire target 103, up throughthe error consolidator 42. However, the reference database does notdescribe an idealized fiducial mark pattern, but describes the areas ofthe target where the fiducials are expected, as being clear areas devoidof any pattern. Thus, when the reference data is compared with the scandata, the fiducial marks register as errors. The error consolidator 42outputs the consolidated error description of the top fiducial mark 518as before, i.e., the area in pixels, X maximum, X minimum, Y maximum, Yminimum and the type of error. The error consolidator treats the bottomfiducial mark 519 and any other pattern in the scanned areas, in thesame manner as for the top fiducial mark 518. As before, DPC1 56 sortsthe errors in descending order of pixel area, creates a list of theerrors, and sends the list to host computer 3.

Host computer 3 compares each of the errors with the following test:

    __________________________________________________________________________    IF AREA<=MAX --FIDUCIAL --AREA AND                                            IF AREA>=MIN --FIDUCIAL --AREA AND                                            IF (X maximum-X minimum)<=FIDUCIAL --WIDTH+(2*TOLERANCE) AND                  IF (X maximum-X minimum)>=FIDUCIAL --WIDTH-(2*TOLERANCE) AND                  IF (Y maximum-Y minimum)<=FIDUCIAL --HEIGHT+(2*TOLERANCE) AND                 IF (Y maximum-Y minimum)>=FIDUCIAL --HEIGHT-(2*TOLERANCE)                     __________________________________________________________________________

THEN a fiducial mark has been found and corresponds to the data tested.The parameters MAX₋₋ FIDUCIAL₋₋ AREA and MIN₋₋ FIDUCIAL₋₋ AREA are themaximum and minimum fiducial areas, respectively, and they describe theacceptable limits within which a fiducial mark must fall. ParametersFIDUCIAL₋₋ WIDTH and FIDUCIAL₋₋ HEIGHT are the nominal width and nominalheight, respectively, of an ideal fiducial mark. 2* TOLERANCE is aparameter setting the acceptable deviations from the ideal width andheight for a valid fiducial mark. The parameters are unique for eachfiducial mark. Preferably, Top 518 and bottom 519 fiducial marks havedifferent shapes and the parameters for the two will not be the same,although the parameter TOLERANCE preferably is the same for both. Theabove test is performed on each of the errors, first with the parametersfor the top 518 fiducial and then with the parameters for the bottom 519fiducial.

If the two fiducial marks are within the scanned areas of the target,and if they fall within the parameters defining acceptable fiducialmarks, then the host computer will identify which of the fiducial marksis the top fiducial mark 518 and which is the bottom fiducial mark 519.

If the top 518 and bottom 519 fiducial marks have been locatedsuccessfully, then their centroids will be determined by the hostcomputer 3. Since the shapes chosen for the fiducial marks aresymmetrical about their centroids, the X-Y coordinates of the centroidsare determined by host computer 3 by setting X equal to X minimum+((Xmaximum-X minimum)/2), and setting Y equal to Y minimum+((Y maximum-Yminimum)/2).

If the top 518 and bottom 519 fiducial marks have the same Xcoordinates, then the target is aligned already. If the X coordinate ofthe top fiducial mark 518 is less than the X coordinate of the bottomfiducial mark 519, then the stage 35 has to be rotated clockwise toalign the target 103. If the X coordinate of the top fiducial mark 518is greater than the X coordinate of the bottom fiducial mark 519, thenthe stage 35 has to be rotated counter-clockwise to align the target103. The stage will be rotated about the center of the target by anamount equal to one-half the absolute value of the difference betweenthe X values of the top 518 and bottom 519 fiducial marks, as measuredat the top or bottom fiducial. Preferably, the centroids of thefiducials are equidistant from the center of the target 103, and arecentered in the X direction on the target 103.

The inspection process then can be carried out again on the fiducialmarks to see if the centroids are within half a pixel of each other inthe X direction. If they are not, then again the stage can be rotated.This process can be carried out as many times as are necessary toachieve the desired accuracy of alignment.

The distance MD between the centroid of the top fiducial mark 518 andthe centroid of the bottom fiducial mark 519 is computed by hostcomputer 3 by finding the absolute value of the difference between the Ycoordinates of the two centroids. The measured distance MD is comparedagainst a predetermined value PD for the distance. There is apredetermined value PV of the distance between two successive Yaddresses, that corresponds to the predetermined value PD of thedistance between the two centroids. The actual distance AV between twosuccessive Y addresses that corresponds to the measured distance MDbetween the two centroids, can be found by solving for AV in thefollowing relationship: (PD/MD)=(PV/AV). The actual distance AV is thespacing to be used between two adjacent rows 120 for the then existingenvironmental conditions for the target 103. These environmentalconditions include the ambient temperature for the target, whichinfluences the value for the measured distance MD.

Semiconductor Wafer Bar-by-Bar Alignment

The invention comprehends direct laser writing and inspection ofintegrated circuit patterns on the individual chips 526, i.e., IC bars526, of a semiconductor wafer 525, as seen in FIG. 46. The wafer writingprocess is carried out by laser exposure of photosensitive photoresiston the face of the wafer. The inspection process is carried out onpatterns of etched photoresist or etched metal on the semiconductorwafer.

Typically, each semiconductor wafer 525 is comprised of a plurality ofrectangular IC bars 526, usually arranged in a rectangular matrixformat. The IC bars will be defined by a grid of scribe alleys 529. Asingle IC bar 526 is defined by the area of semiconductor materialbetween two adjacent, parallel, horizontal scribe alleys and between twoadjacent, parallel, vertical scribe alleys.

If only a single patterned layer is required for each IC ar, requiringonly a single raster scanning of the areas of the IC bars, thenwafer-level alignment marks may be sufficient to determine the Xpositions marking the beginning of the rows 120 of the various IC bars.Wafer level alignment marks are marks uniquely identifiable by thepattern inspector 400 when in the inspect mode, as described previouslyin connection with autoalignment. The alignment marks need only be twoin number to establish X and theta offsets of the orientation of thewafer, with respect to the orientation required to produce scannedstrips 105 that are parallel with the vertical scribe alleys.

For more demanding applications, where previous layers have beenpatterned using bar-level alignment of optical wafer steppers,bar-by-bar alignment is required in addition to the wafer-levelalignment discussed previously under autoalignment. Bar-by-bar alignmentis not required in those instances where the first or only patterninglayer is patterned by the laser pattern inspection system 400 while inthe writer mode.

FIG. 46 shows a section of a target 103 which is a semiconductor wafer525, having a plurality of IC bars 526 on which integrated circuits maybe fabricated. The IC bars are separated by a rectangular gridwork ofintersecting rows and columns of scribe alleys 529. Scribe alleys arethe areas which will be cut away or otherwise destroyed during theprocess of separating the individual IC bars 526 after the integratedcircuits have been fabricated on the IC bars. Each IC bar 526 hasassociated with it an upper bar-alignment-mark 527 and a lowerbar-alignment-mark 528, which were patterned photolithographically inthe vertical scribe alley 529 to the left of the IC bar. Thebar-alignment-marks were patterned at the same time that the firstcircuit layer was patterned on the IC bars 526. The bar alignment marksare illustrated as crosses, however, any recognizable pattern wouldwork. Two alignment marks per bar have been shown, but more than two canbe used.

Each pair of upper 527 and lower 528 bar-alignment-marks is associatedwith the IC bar immediately to its right and is at a predeterminedorientation and distance from the pair. Each pair of bar-alignment-marksdefines the rotational and translational offsets of the X-Y coordinatesystem for the previous layer or layers of patterning for that IC bar526, with respect to the wafer-level orientation.

The inspection system 400 identifies and locates the bar-alignment-marksby raster scanning only the vertical scribe alleys 529. The verticalscribe alleys are scanned in strips 105 extending the full length of thescribe alleys. By scanning only the scribe alleys, time is saved overscanning the entire wafer 525. The scan data from thebar-alignment-marks can be compared to a reference data representationof the bar-alignment-marks, or, as in the case of autoalignment,described previously, the list of data corresponding to thebar-alignment-marks is sent to host computer 3 where the offsets aredetermined and an IC bar location map is constructed. Each scribe alleyrequires only a low number of scanned strips to identify and locate allthe bar-alignment-marks, further reducing the total wafer scan time. Astepping map created by the optical stepper while patterning the firstlayer, combined with the individual bar header information or steppingdesign information can be used to provide the locations for each of thebar level bar-alignment-marks. Instead of the bar-alignment-marks,isolated, prominent features of the first layer of pattern, providingrecognizable orientation information can be used.

Either the wafer 525 must be rotated and/or translated, or the laserpattern inspection system 400, while in the writer mode, must compensatefor the X-Y translation by timing the beginning of modulation of thescanning laser beam and/or by rotating the data from the pipelines. Thepreferred embodiment of the invention contemplates using the lattermethod. Likewise, during inspection of the wafer 525, the top 527 andbottom 528 bar-alignment-marks are used to determine the individualorientations of the IC bars 526 so that data from the phototube 45 willbe clocked into ECL comparator 40 at the correct timing offsets so thatthe scan data will correspond to the correct pixel reference datapositions.

Since most of the IC bar 526 pattern offsets from the wafer-levelorientation are translational and not rotational, it should besufficient in the majority of instances to compensate for the offsetssolely by timing when to begin raster scanning each row. That is, theX-Y position of the start of each row is determined in light of the Ystage motion and X sweep direction by timing when to begin a sweep andtiming at what point in the sweep that modulated data should begin to beoutput.

The wafer 525 is written or inspected by scanning the wafer in stripsextending nearly the full height of the wafer, rather than by scanningeach IC bar, one-at-time, in strips extending only for the height of theIC bar 526. This saves turnaround time at the end of each strip by adivision factor equal to the number of IC bars 526 on the wafer 525.Each strip has a height sufficient so that the aggregate of the stripscover all the areas on the wafer which require patterning.

Since each wafer 525 is raster scanned in strips extending for theheight of the wafer, strip memory buffers can be used at the outputs ofthe pixel memory modules 24A-24D to store a strip's worth of translatedand possibly also rotated data to be written or used as reference dataduring the inspection process.

Autofocus for Writer and Inspection Systems

The autofocus system allows for very fast laser writers 50 andinspectors 400 to be able to move across non-flat targets 103 at highspeed and still keep the objective lens 117 in focus. A small,preferably solid-state, laser is used to direct a focused spotcoinciding with the spot focus of the objective lens on the surface oftarget 103. That is, both the writing or inspection laser 100 and theautofocus laser direct laser beams to the same pixels location on target103. The autofocus spot is picked up as a reflection separate from anyreflections from the scanning laser beam, and relayed to a linearphotodetector which determines the target 103 surface to lens height bythe lateral distance apparent to the detector. This information is fedto a spiral or ring and magnet system to keep the planametric distanceconstant.

For one embodiment of the laser writer system 50, an objective lens 117having a fixed reduction of 7.145 and working over a distance of 23inches was required. The depth of focus of this lens was less than onemil, but target 103 surface variations can cause theobjective-lens-117-to-target-surface distance to vary as much as severalmils. The autofocus system corrects for this distance variation. Thehigh speeds of stage 35 require that the autofocus system be able tocompensate for focusing variations up to the rate of 100 hz. Anelectro-mechanical syste moves just the lower portion of themulti-element objective lens 117, with external paralactic sensing ofthe focal error. The focusing variations required to follow the surfacecontour of the target 103 do not change the reduction ratio of theobjective lens 117. system 117 includes an infinity corrected highresolution objective lens 501 movable along the optical axis to followthe surface contour of the target 103 to maintain focus of the laser 100on the surface of target 103. The objective lens 501 movement along theoptical axis takes place in a region 502 in which the laser beam iscollimated, as indicated by parallel light rays 500.

The intermediate, non-moving long focus lens system 503 sets themagnification and track length. The upper field lens 504 gives the laserdeflection system telecentricity. The laser deflection system includesthe chirp deflector 102.

A 2 mw, 820 nm, 5 mm diameter collimated beam, gallium arsenide,infra-red laser 505 is used to provide the autofocus spot laser lightfor tracking the objective-lens-117-to-target-surface distance. The 100mm lens 506 is used to focus the collimated light from laser 505 and toset the focal point of laser 505 at the desired writing or inspectiondistance. This distance is the objective-lens-117-to-target-surfacedistance. Prism 507 directs the laser beam from autofocus laser 505 tostrike the target 103 surface coincidentally with the same spot struckby writer/inspector laser 100 beam. The prism 507 directed laser beam508 has an angle of incidence with respect to the target 103 surface, of45 degrees, and is specularly reflected from the target surface at areflected angle of 45 degrees into a second prism 509. The second prism509 directs the reflected autofocus 514. The directed, reflected laserbeam directed by the second prism 509, is parallel to the optical axisof the objective lens 117 only if theobjective-lens-117-to-target-surface distance is correct.

The first of the series of two lenses is the 100 mm collimation lens510, which re-establishes collimation of the reflected autofocus laserbeam. The recollimated beam which emerges from the collimation lens 510,is exactly parallel with the optical axis of the objective lens 117 onlyif the objective-lens-117-to-target-surface distance is the desireddistance. If the distance is too large, then the recollimated beam isdeflected to the right, as seen in FIG. 44. If the distance is toosmall, then the recollimated beam is deflected to the left, as seen inFIG. 44.

The second of the series of two lenses is the telescope objective lens511, which focuses the recollimated beam into a small spot on thesilicon photodetector 514. The distance 512 between the collimation lens510 and the telescope objective lens 511 is chosen to be approximatelyequal to the focal length of the collimation lens 510. This makes allreflected beams pass centrally through the telescope objective lens 511,notwithstanding any aperture shearing at the collimation lens 510 due tomain focal error.

In order to prevent stray or diffusely reflected laser light from thewriting/inspection laser 100 from reaching the photodetector andinterfering with its operation, an optical filter 513 is placed in frontof silicon photodetector 514. Some embodiments of the invention utilizea writer/inspector laser 100 which emits blue-green light at a 488 nmwavelength. In that embodiment, the optical filter 513 will preferablybe a red filter for filtering out light at a wavelength of 488 nm.

The silicon photodetector 514 is a split "Double-D" design,differentially connected to a servo-amplifier system for moving axiallythe high resolution objective lens 501 to keep the writing/inspectionlaser beam focused on the surface of the target 103. Anyobjective-lens-117-to-target-surface distance errors are translated intohorizontal laser 505 spot motions across the photosensitive face of thesilicon photodetector 514. The spot motions across the "crack", or splitdiode, in the face of the photodetector 514, give a plus or minus outputfrom the photodetector 514, that is approximately linear over a spotmovement range large enough to cover any normal variations in thecontour of the target 103. A screw 515 is for adjusting the horizontalposition of the mechanical stage upon which the photodetector 514 ismounted. With the screw 515, the horizontal position of thephotodetector can be adjusted to make zero electrical output from thephotodetector correspond exactly to the desired writing/inspectionfocus. That is, the desired objective-lens-117-to-target-surfacedistance exactly corresponds to zero electrical output from thephotodetector.

The objective-lens-117-to-target surface distance error discovery systemis based on parallax. The autofocus system responds primarily to thevertical position of the target 103 surface, and is not responsive tosmall angular variations of the contour of the tested spot on the targetsurface. The autofocus system is configured to be tolerant of minoroptical imperfections in its component parts.

A two-section differentially connected coil 516 of 520 turns of number26 wire, and having a total electrical resistance of 12.5 ohms, is fedelectrical currents of up to plus or minus 0.7 amps by the servoamplifier (not shown). A one inch long 60 gram cylindrical magnet 517 ofmoment 2500 cgs units, responds to the coil current to produce therequired axial motions of the moving assembly. The moving assemblyincludes the high resolution objective lens 501, the cylindrical magnet517, the prisms 507 and 509, the 100 mm focusing lens 506, and thecollimator lens 510. This moving assembly is supported by 0.008 inchthick beryllium-copper spiral-cantilever springs 518. The movingassembly weighs 280 grams and resonates at 28 hz with the spiralcantilever springs. Besides this primary resonance, there are severalsecondary resonances, of which the most important is a very high-Qresonance at 900 hz. Motions with amplitudes up to plus and minus 0.010inches are possible.

Pattern Inspector Data Flow

The data flow for the pattern inspection system 400 is illustrated inFIGS. 53A and 53B, and is very similar to the data flow for the patternwriter system 50, as discussed previously in connection with FIG. 9.FIGS. 53A and 53B differ from FIG. 38 in that FIGS. 53A, 53B only showtwo pipelines 322 and 323, whereas FIG. 38 shows an inspection system400 having four pipelines. As has been indicated previously,theinvention comprehends the use of two or more pipelines to accomplishparallel processing of the database to a bit-map final form. FIGS. 53A,53B shows only two pipelines merely to simplify the diagram.

The tape 17 resident database representation of a pattern is the samefor the inspection system 400 as for the writer system 50. Thus, for theexample rectangle 300 pattern shown in FIG. 26, the database 313 shownin FIG. 27 is also the database for rectangle 300 during an inspectionprocess carried out as shown in FIGS. 53A, 53B.

As with the writer system 50, DPC2 reads the database from tape 17,reformats it into a turnpoint polygon representation, and stores thatrepresentation on data disk 13. What is different from the writer system50, however, is that with the inspection system 400, DPC2 also addsguardband polygon descriptions to the database. These guardband polygonsare also described in turnpoint polygon representation. For purposes ofsimplicity, only the reference data and guardband polygon data for frame301 is shown, since the data for frame 302 is constructed in just thesame way.

The reference data for frame 301 is identical to the reference datashown in FIG. 29 for the writer system 50.

In FIG. 48A, the data indicated at 254 is the turnpoint description ofguardband 249 shown in FIG. 47. Likewise, the data indicated at 255 isthe turnpoint description of guardband 250, and the data indicated at256 is the turnpoint description of guardband 248. As can be seen,reference polygons and guardband polygons are each described separately.

FIGS. 49A and 49B illustrate the guardband data only which is sent downpipelineB at 323. The reference polygon data processing in the pipelineswill not be shown since it is just the same as was illustrated anddiscussed with reference to FIG. 9 and writer system 50. The 25 bitwords for the guardband polygons are constructed in the same manner asfor the reference polygons.

FIGS. 50A and 50B illustrate the guardband data as formatted bypreprocessor2 20B of FIG. 53A. The guardband polygon data has beenconstructed in the same manner as for the reference data.

FIGS. 51A through 51C illustrate the guardband data as formatted byfiller module2 23B of FIG. 53A.

The first 25 bit word of FIG. 51A has a 5 bit command field 342 equal to1C hex, which indicates that in the pixel memory module, the guardbandpolygon data is to be logically ORed with the data already at thatmemory location. Since the pixel memory module starts with all itsmemory positions cleared to zero, this allows overlapping guardbandinformation to be loaded into pixel memory. The second word begins withcommand field 342 which is equal to 07 hex. This indicates that theguard band polygon data bits 000007 hex will be loaded into memory andindicate three ones are to be set from X=1FD hex to X=1FF hex at Y=0FDhex. The following two 25 bit words set the four bits from X=200 toX=203 at Y=0FD hex. Thus, the seven bits which are on or in the interiorof guardband polygon 249 at Y=0FD hex have been set by the first four 25bit guardband data words. Similarly, the next four 25 bit guardband datawords set the seven bit for guardband polygon 249 at Y=0FE hex, and soon down to the last Y address, Y=1FF hex for guardband polygon 249.

FIG. 51B illustrates the setting of the bits on or within guardbandpolygon 250. Here, eight 25 bit words are required since guardbandpolygon 250 extends from X=0FD hex to X=203 hex.

FIG. 51 C illustrates the setting of the seven bits at each X address ofguardband polygon 248, in a similar manner to what is illustrated inFIG. 51A for guardband 249, since they both have the same width in the Xdirection.

FIG. 52 illustrates the 64 bit word bit-map for the guardbands that areconstructed by pixel memory module 24B, in a similar manner to thatdiscussed in connection with FIG. 37 for the writer system 50.

As previously discussed, the guardband data from pixel memory module 24Bis logically ANDed bit-at-a-time with the result of logicallyExclusively ORing the corresponding bit from the scan data 257 with thecorresponding bit from the reference data from pixel memory module 24A.

We claim:
 1. An autofocus system for a pattern inspection or writingsystem, comprising;an objective lens assembly including a fixed longfocus lens and a movable objective lens for focusing a first laser beamon the surface of a target for pattern inspection or writing; anautofocus laser fixed to the objective lens assembly, for directing asecond laser beam to strike the surface of the target at an obliqueangle; a photodetector for detecting the relative movement of thereflected second laser beam, as the distance of the objective lensassembly to the surface of the target varies, and for generating anelectrical signal in response to the relative movement; and aservo-system responsive to the electrical signal, for moving saidmovable objective lens axially to keep the first laser beam focused onthe surface of the target as the distance of the objective lens assemblyto the surface of the target varies.
 2. The autofocus system of claim 1,wherein the said movable objective lens is a constant magnification,variable focus objective lens.
 3. The autofocus system of claim 1, wherein the photodetector is fixed to the objective lens assembly.
 4. Theautofocus system of claim 1, wherein the photodetector is a split,Double-D, silicon photodetector.
 5. An autofocus system for a patterninspection or writing system, comprising;an objective lens assemblyincluding a fixed long focus lens and a movable objective lens forfocusing a first laser beam on the surface of a target for patterninspection or writing; an autofocus laser fixed to the objective lensassembly, for directing a second laser beam to strike the surface of thetarget at an oblique angle; a photodetector for detecting the relativemovement of the reflected second laser beam, as the distance of theobjective lens assembly to the surface of the target varies, and forgenerating an electrical signal in response to the relative movement;and a coaxial linear motor responsive to the electrical signal, forproviding fast response for moving said movable objective lens axiallyto keep the first laser beam focused on the surface of the target as thedistance of the objective lens assembly to the surface of the targetvaries.